Transitions

- CHANGED API: `MachineError` is now limited to internal error and has been replaced by `AttributeError` and `ValueError` where applicable (thanks to ankostis)
- CHANGED API: Phasing out `add_self`; `model=None` will add NO model starting from next major release; use `model='self'` instead.
- Introduced deprecation warnings for upcoming changes concerning `Machine` keywords `model` and `add_self`
- Introduced `Machine.remove_transition` (thanks to PaleNeutron)
- Introduced `Machine._create_state` for easier subclassing of states
- `LockedMachine` now supports custom context managers for each model (thanks to paulbovbel)
- `Machine.before/after_state_change` can now be altered dynamically (thanks to peendebak)
- `Machine.add_ordered_transitions` now supports `prepare`, `conditons`, `unless`, `before` and `after` (thanks to aforren1)
- New `prepare_event` and `finalize_event` keywords to handle transitions globally (thanks to ankostis)
- New `show_auto_transitions` keyword for `GraphMachine.__init__` (default `False`); if enabled, show auto transitions in graph
- New `show_roi` keyword for `GraphMachine._get_graph` (default `False`); if `True`, show only reachable states in retrieved graph
- Test suite now skips contextual tests (e.g. pygraphviz) if dependencies cannot be found (thanks to ankostis)
- Improved string representation of several classes (thanks to ankostis)
- Improved `LockedMachine` performance by removing recursive locking
- Improved graph layout for nested graphs
- `transitions.extensions.nesting.AGraph` has been split up into `Graph` and `NestedGraph` for easier maintenance
- Fixed bug related to pickling `RLock` in nesting
- Fixed order of callback execution (thanks to ankostis)
- Fixed representation of condition names in graphs (thanks to cemoody)

- Support dynamic model addition via `Machine.add_model` (thanks to paulbovbel)
- Allow user to explicitly pass a lock instance or context manager to LockedMachine (thanks to paulbovbel)
- Fixed issue related to parsing of HSMs (thanks to steval and user2154065 from SO)
- When `State` is passed to `Machine.add_transition`, it will check if the state (and not just the name) is known to the machine

- Machines can work with multiple models now (thanks to gemerden)
- New `initial` keyword for nested states to automatically enter a child
- New `Machine.trigger` method to trigger events by name (thanks to IwanLD)
- Bug fixes related to remapping in nested (thanks to imbaczek)
- Log messages in `Transition.execute` and `Machine.__init__` have been reassigned to DEBUG log level (thanks to ankostis)
- New `Machine.get_triggers` method to return all valid transitions from (a) certain state(s) (thanks to limdauto and guilhermecgs)

- `async` is renamed to `queued` since it describes the mechanism better
- HierarchicalStateMachine.is_state now provides `allow_substates` as an optional argument(thanks to jonathanunderwood)
- Machine can now be used in scenarios where multiple inheritance is required (thanks to jonathanunderwood)
- Adds support for tox (thanks to medecau and aisbaa)
- Bug fixes:

- Problems with conditions shown multiple times in graphs
- Bug which omitted transitions with same source and destination in diagrams (thanks to aisbaa)
- Conditions passed incorrectly when HSMs are used as a nested state
- Class nesting issue that prevented pickling with dill
- Two bugs in HierarchicalStateMachine (thanks to ajax2leet)
- Avoided recursion error when naming a transition 'process' (thanks to dceresuela)

- Minor PEP8 fixes (thanks to medecau)

- New `async` Machine keyword allows queueing of transitions (thanks to khigia)
- New `name` Machine keyword customizes transitions logger output for easier debugging of multiple running instances
- New `prepare` Transition keyword for callbacks before any 'conditions' are checked (thanks to TheMysteriousX)
- New `show_conditions` GraphSupport keyword adds condition checks to dot graph edges (thanks to khigia)
- Nesting now supports custom (unicode) substate separators
- Nesting no longer requires a leaf state (e.g. to_C() does not enter C_1 automatically)
- Factory for convenient extension mixins
- Numerous minor improvements and bug fixes