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.
I was wondering actually, should we use `@Nullable`/ `@NotNull` in this library? If so, which implementation? We use the JetBrains one at SoundCloud, because I believe at some point we found that the `javax` annotations crashed certain Samsung devices. I would just prefer to document expectations around nullables as part of the signature, not hidden away with a runtime error.