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). 
Perhaps it would make more sense, on the other hand they serve as a "gentle introduction". This example is a direct translation of the current [asynchronous observable examples](https://github.com/Netflix/RxJava/wiki/How-To-Use-RxJava#asynchronous-observable-example) from the wiki.