Support 'unbinding'
Generate android.content.Context stub if not in classpath

Resolves exception caused reflecting on `com.firebase.client.Firebase`
without having the android SDK JARs in your classpath.
Adjust `bind` to return channel AND unsubscribe function
Registry to simplify closing listeners
Test watch changes to library code too
Merge pull request #20 from verma/feature/unbind

Support unbinding event listeners