Add a DemoApplication and RecyclerViewDemoActivity as sample

[htcbug]

@puf

Hi Frank,

Please review this change. It adds a DemoApplication and a RecyclerViewDemoActivity. After this change I'll add more code using the FirebaseRecyclerViewAdapter and merge my changes into it.

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[puf]

@htcbug Any idea why these lines are needed?

[puf]

@htcbug overall it looks good, but I have a few comments. Address those (either in code or by responding) and assign the PR back to me.

[puf]

I'm going to merge this in to master. Any more changes will end up there or on a new branch.