Update all links to Bevy docs to point to v0.6.1, and add to `README.md`
the missing predefined rotation lenses.

Add more predefined rotation lenses interpolating the angle of rotation
instead of the `Quat` itself directly. Document at the `lens` module
level the difference between the shortest-path lens and the
angle-focused ones.

Bug: #5

Fix some bug lerp'ing colored lenses (`TextColorLens`,
`SpriteColorLens`, `ColorMaterialColorLens`) where the alpha value would
be too high. This only affects non-opaque colors, in that for opaque
blending the resulting alpha was `3.0` which was interpreted as opaque.

Add code coverage via coveralls.io

- Add `TweenCompleted::user_data` to help identify the tween which
  raised the event.
- Improved several docs.
- Update CHANGELOG.
- Update README with mention of events and callbacks.

Add a mechanism to `Tween<T>` to raise a Bevy event once the tween
completed. The event raising can be enabled with `set_completed_event()`
and `with_completed_event()`. The `TweenCompleted` event contains the
`Entity` on which the `Animator<T>` or `AssetAnimator<T>` owning the
tween are attached.

Update the `sequence` example to show how to use that new event
mechanism.

Bevy 0.6.1 was released which fixes nightly builds. Re-add nightly
toolchain to CI and remove the disclaimer from the README.

Provide the callback owner with the `Entity` the tween and the animator
are attached to, as a parameter to the callback when invoked.

Add some new methods and refactor some existing ones to clarify the
playback state of a tweenable, and give increased control to the
`Animator` or `AssetAnimator` to rewind a tweenable, set its progress to
an arbitrary value, or query its current state.

This avoids having those predefined types, which are optional, be in the
middle of the core types.

Ensure the `TweenState::Ended` is set only for `TweeningType::Once`,
which is the only one with a finite duration, so which can end. Other
tweening types repeat indefinitely until stopped, so have no end.

Bevy 0.6.0 is broken for nightly, which breaks all `docs.rs` builds.
Remove nightly CI and add a README warning until this is fixed in 0.6.1.

Add a `Tweenable<T>` trait describing a generic animatable tween-like
element.

Expose the three types of tweenables and implement `Tweenable<T>` for
them:
- `Tween<T>`, a single animation
- `Sequence<T>`, a sequence of consecutive animations
- `Tracks<T>`, a batch of animations running in parallel

Clean-up animators to hold a single top-level `Tweenable<T>` and let the
user build any kind of animation hierarchy for themselves.

Allow querying the active tween of a sequence. Use this to improve the
`sequence` example with some text showing the current active tween and
its progress.

Implement some callbacks invoked when a tween anim starts or ends.

Export publicly the asset and component system to allow an app to
manually add them.

Add CI