reactivex.io: RxScala getting started examples

This PR adds a new examples folder called `reactivex` to RxScala. The idea is to embed these examples directly from the repo in the reactivex.io website.

Contains (runnable) Scala versions of the examples used in [how to use](https://github.com/Netflix/RxJava/wiki/How-To-Use-RxJava). 
Wouldn't it make more sense for these async examples to use subscribeOn, instead of a Thread/Executor/ThreadPool, to simplify the code and show how RxScala can help you. Or is that not a pattern we want to promote?