Refactor toMap

A PR to refactor `toMap`.

There is a breaking change in this PR: 
* Remove `def toMap[K, V] (keySelector: T => K, valueSelector: T => V, mapFactory: () => Map[K, V]): Observable[Map[K, V]]`

The other changes are still source compatible and binary compatible.
If you remove this example, you should show how we can do this now. Not necessarily with a non-empty starting map, but maybe with a HashMap from the collection library?