New project structure based on submodules (added samples module)

So, I finally got around to doing this. There was another PR open (#10) which turned into a dead end, since there were plugin incompatibilities between the stuff that RxJava/Nebula pulls in and the Android Gradle plugin.

This is the second attempt, in which I used my "build wrapper" hack again that I used for the original rxjava-android module back when it was part of core. I know it's not pretty, but at least it works. What it does is create two submodules for every Android project (like samples, or integration tests), the actual module and a build wrapper. The build wrapper becomes a submodule of the root project, and when it receives the build command it forwards it to a "sub-Gradle" in the actual target module. That way all the Android projects that are submodules can be completely self-contained sandboxes and have their own Gradle wrapper, so no plugins can clash.

I do share the library versions (project and core libs) via the root properties, by having the wrapped modules load and parse the root properties file :-/

Long story short: this will finally enable us to have
- a samples submodule that builds with the root project and can be imported separately into Android Studio (already included in this PR, although the samples need to be updated...)
- an acceptance test submodule to run Android instrumentation tests on real devices against RxAndroid, so as to discover regressions more quickly
- a framework library submodule to contain the work @Dorvaryn has started

Let me know what you think!
That's not to say there might be nicer ways to achieve the same thing. Please do let me know if you find any!