Added LifecycleObservable binds

This is the first half of the puzzle that is #12. It provides two main facilities:

- `LifecycleObservable.bindUntilLifecycleEvent()` lets you specify an exact point in a lifecycle to unsubscribe at.

- `LifecycleObservable.bindActivityLifecycle()` and `bindFragmentLifecycle()` which make a best effort guess at when you want a subscription to end, based on the lifecycle state.

Implementation detail: `OperatorSubscribeUntil` acts like `takeUntil()`, except that it unsubscribes instead of calling `onComplete()` when the conditional `Observable` emits an item.

If this works then we can later figure out how to create these `Observable<LifecycleEvent>` streams.
Agreed that `@Nullable` and `@NotNull` would be great.

I think it'd make more sense in another PR. In this PR it would either be too limited in scope (to just `LifecycleObservable`) or it would make the PR too broad (if I fixed everything).