RxScala page migration

This PR contains the migrated content from the [rxscala.github.io](rxscala.github.io) website (including scaladoc).
The only problem is that the generated comparison table currently isn't rendered correctly by our Jekyll instance. We either have to configure it to work or update the [generation code](https://github.com/ReactiveX/RxScala/blob/0.x/src/test/scala/rx/lang/scala/CompletenessTest.scala).