Rx

- Extension methods and extension class methods have been removed. This
makes it much easier for editors and IDEs to validate the code and
perform code completion.
- Python 3.6+ only with type hints
- Google docstring style.

- Refactored virtual time scheduling. Fixes 95. Thanks to djarb
- Fixed Visual Studio project files and moved to ide folder.
- Remove timer operations from base `SchedulerBase` class.
- Scheduler.now is now a property to align with Rx.NET
- Bugfix for periodic scheduling. Fixes 91. Thanks to frederikaalund
- Demonize all threads in `TimeoutScheduler`. Fixes 90
- Enable subscription with duck-typed observer.
- Added new core module. Observable, Observer, Scheduler and Disposable
are now ABCs.
- Synced backpressure with RxJS to fix 87
- Do not overwrite scheduler keyword arg. Fixes 85. Thanks to rjayatilleka
- Added async iterator example.
- Added support for awaiting observables
- Fixed issue 83 with `int + datetime.datetime` in timer.py. Thanks to AlexMost

- Fixes for TwistedScheduler raising AlreadyCalled error 78. Thanks to
mchen402 and jcwilson.
- Use CurrentThreadScheduler as default for just/return_value. Fixes 76

- Added wxscheduler.py for use with wxPython applications thanks to
bosonogi
- Added eventletscheduler.py for use with Eventlet thanks to jalandip
- Protect generators that are not thread safe. Fixes 71

- Threads are now daemonic by default. Thus they will exit if parent
thread exits which should probably be what most people want.
- Fix for recursive scheduling, thanks to smeder
- Fix for NewThreadScheduler. Now uses EventLoopScheduler to make sure
scheduled actions by scheduled actions happens on the same thread.
- Uses shields.io to uniformize and fix the badges, thanks to DavidJFelix

- Fix optional parameter in `delay_subscription`. Thanks to angelsanz.
- Simplified `adapt_call` in `util.py` which makes higher order functions
accept more forms of callables.
- Fix for Python 2.7 in `timeflies_qt.py`.