Application setup and message composition layout

Just a basic application setup. Also, includes a "Message composition"
layout to serve as a runnable app mockup.
Add README and license
Add rxjava-android and experimental helpers

This helpers are not final API, but an experiment with fluent API for
binding `Observable` to Android views.
Message composition logic using only reactive observables

Our `Activity` code is pretty much declarative, condensing all of our
logic on our `onCreate`, instead of disperse event callbacks.

The downside is the "submit" action, where composing the final message and
sending is not intuitive as I first expected.

But on other side, our responsive UI code really shines! :smile: