Mopidy-alarmclock

- Cosmetic improvements.

- Upgraded to Mopidy 3.0+ and Python 3.7+.

- Play [backup alarm sound](http://soundbible.com/1787-Annoying-Alarm-Clock.html) when playback cannot be started (within 30 seconds or more).
- Added warning to readme that only local playlists/media should be used for alarm clock.

- Changed branching model to [git-flow](http://nvie.com/posts/a-successful-git-branching-model/).
- Refactoring for improved alarm scheduler.
- Added [backup alarm sound](http://soundbible.com/1787-Annoying-Alarm-Clock.html) (in case selected playlist is missing).
- Disable _Consume_ and _Single_ playback modes.
- Fixed incorrect Mopidy version requirement.
- Misc refactoring.

- Added tests.
- Fixed nondeterministic effects when cancelling and setting alarm again within 5 seconds (prevent stale `idle()` timers).
- Fixed minor math bug in gradual volume increasing.
- One digit minutes supported in alarm _Time_.
- Leading zero for hours of current time in _Alarm state_.

- Alarm defaults can now be configured in `mopidy.conf`.
- Display alarm volume on _Alarm state_.
- Display current time of alarm clock on _Alarm state_.
- Added [Travis-CI build](https://travis-ci.org/DavisNT/mopidy-alarmclock) and [Coveralls test coverage info](https://coveralls.io/r/DavisNT/mopidy-alarmclock).
- Fixed README (to be parsable by PyPI).