A more flexible database

• Added support for sorting/transforming the results of Firebase queries before they are passed to any data sources.
• Decoupled data sources from Firebase collections, so you can spin your own.
• Added support for subclassing to FUIArray. It is no longer painful to make custom collections that have their own behavior (see #186).
• Added methods arrayDidBeginUpdates: and arrayDidEndUpdates: to FIRCollection so consumers can batch changes more easily.
• Added observeQuery and invalidate methods. FUIArray no longer observes upon initialization and requires a call to observeQuery before emitting any events.

These are breaking API changes, so FirebaseUI is now in 2.0.0.