Extra observables

I have been using RxJavaFX for a couple of months now and often came across limitations of this library. To solve these limitations, I added a number of wrappers that must be in this library to support the collaboration between RxJava and JavaFX more optimally.

First of all, besides multiple JavaFX has a number of observable collections: `ObservableList`, `ObservableMap` and `ObservableSet` that can be turned into Rx Observables. Every time a collection fires a change event, a new snapshot of that collection is put into the Rx Observable.

Secondly, the RxJavaFX library was already capable of fetching all events of a certain type fired from a specific `Node`. However, this was not possible for something like a `Stage` or other type of `Window`. Therefore I added a wrapper that supports fetching a certain type of events from a `Window`.

Let me know what you think about this and what improvements there could be on the new code.