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.

This was the "lightest" hack I could come up with, but not sure if it would "just work" transitively when in user projects with Pani as a dependency. Also would need to warn users about needing to remove the stub class (`lein clean` most likely) if deciding to include Android as a dependency later.

@verma thoughts?