Lemoncheesecake

- **lcc run**: when the user hits Ctrl-C, every already running tests will be interrupted when/if
the test calls a lemoncheesecake logging function
- Fix test skip reason not properly set under certain circumstances
- **under the hood**: various main code (among which `lemoncheesecake.session`) and test code refactoring

- **API**: the `detached_step` context manager (whose purpose is to create steps in new threads) has been deprecated;
the `set_step` function can now be used in any context (main thread or new threads)
- **API**: the `detached` argument of the `set_step` is now deprecated
- **CLI**: the `at_each_event` option of `lcc run --save-report` has been renamed into `at_each_log`
(backward-compatibility has been preserved)
- **lcc report**: improve logged URL display
- **doc (http://docs.lemoncheesecake.io)**: add a "Deprecations" section
- **under the hood**:
- the step handling has been totally reworked internally in order to:
- remove special case of "detached steps"
- improve step handling when dealing with multiple threads within the same test
- provide a real "end of step" event
- `lemoncheesecake.exceptions` cleanup:
- reorganize exception handling
- reduce the number of exception classes from 19 to 13
- improve the naming of various exception classes

- **ReportPortal**: enforce reportportal-client version ~= 3.0 to avoid
upgrading to the non-compatible 4.0.0 version

- **ReportPortal**: do not send steps and setup/teardown results to ReportPortal if they are empty
- Fix logging in a new thread when no explicit step has been set
- **under the hood**:
- hold events related to step and setup/teardown in to order to avoid empty steps
and empty results as soon as possible in the event processing chain
- put ReportPortal reporting backend under unit tests

- Fix exponential tests running time when dealing with very large test suites (thousands of tests)
- **under the hood**:
- put Slack reporting backend under unit tests
- refactor Slack reporting backend
- refactor tests of JSON & XML reporting backends and events replay
- in behave integration unit tests, avoid forking a behave process in order to speed-up tests and
make test coverage work

- In `lcc report -s` and HTML report, fix wrong success percentage with in progress tests
- **under the hood**: refactor statistics computation
- **doc & packaging**: update the lemoncheesecake's tagline for "Test Storytelling"