More formal support for Scala

[mattrjacobs]

I'm still in the middle of fleshing this out, but roughly:

1. Removal of all Object methods from rx-core
2. Scala RxJava usage doesn't depend on runtime class lookup - instead it uses some implicits from Scala function types to Rx Func1, Func2, ..., FuncN to assert type safety while using native Scala closures
3. Unit tests to demonstrate full coverage of Observable methods using Scala

Given the first point above, this will have to be combined with #204, so that support for dynamic languages continues.

[mairbek]

Small addition.
For-comprehension for observable should be supported as well.
It means that Observable should have flatMap, map and withFilter methods.

[mattrjacobs]

Good idea. At first glance, I think this could be accomplished by the introduction of an implicit class (ScalaObservable?) wrapping Observable that implements flatMap, map, and withFilter by delegating to mapMany, map, and filter, respectively. As long as all the methods still return an Observable, then we should be OK.