Initial commit of the Android samples module

refs https://github.com/soundcloud/RxJava/issues/3

I made it so that the samples project will automatically be pulled in as soon as it detects an Android SDK on the build machine; otherwise it's ignored. It also runs in a sub-gradle so should not be affected by changes to the main project. It does, however, automatically pull in and compile against the current project version.

Let me know what you think. I kept the actual samples minimal for now, I see this more as a tracer bullet for how the samples could fit into the current build setup.