Edera

* Lowered the minimum required version of `PyMongo` down to 3.1

* Introduced `MonitoringWorkflowExecuter` and `edera.monitoring`:
- Allows workflow executors to report information about tasks
- Allows you to aggregate this information over sources and time, and render it as an HTML
* Added a separate "offline" workflow to the daemon (useful to run monitor watchers)
* Re-worked the `Storage` interface
* Replaced DOA `ZooKeeperStorage` with more efficient MongoDB-based `MongoStorage`
* Refactored duplicating tests (`pytest` is really full of magic)
* Reorganized tests that involve external services (like ZooKeeper or MongoDB):
- Such tests became optional
- No test harnesses - only real service instances
- Now you need to redefine corresponding environment variables in `tox.ini`
* Simplified class factory declarations (like `Qualifier`s)
* Fixed a couple of minor bugs
* Fixed documentation

* Introduced `Storage`s to store versioned key-value pairs:
- `SQLiteStorage`
- `ZooKeeperStorage`
* Converted some workflow executers into workflow processors
* Implemented target caching and integrated it into the daemon
* Fixed various minor bugs

* Introduced hackable `edera.daemon` that allows to run tasks in parallel as a service
* Reworked internal coroutine class hierarchy
* Added lots of `__repr__` implementations to facilitate debugging
* Redesigned qualifiers and added more useful qualifiers:
- `Optional`
- `Date`
- `DateTime`
- `TimeDelta`
* Introduced non-in-place coroutine context extensions using `+`
* Added a new `follow_all` requisite
* Settled an exception handling convention:
- `ExcusableError`s should lead to `ExcusableError`s
- `SystemExit` is not an error - never panic
* Introduced a couple of managers:
- `ZooKeeperManager`
- `CascadeManager`
* Introduced `ManagedInvoker` to run stuff within a context manager
* Discovered a bug related to forking in Python (see `ProcessWorker`)
* Introduced static default parameter values
* Hid phony tasks (ones that don't override `execute`) from the workflow execution log

* Re-implemented `DirectoryLocker` using `sqlite3`
* Removed `fasteners` from the dependencies
* Replaced explicit polling with coroutines
* Introduced a stable asynchronous signal handling agent

* Moved packaging tools from an external repository here