Add CompositeDisposable and change Observer to implement it

[Dave]

This has some nice results.

1. can remove the ondispose callback and the additional closure allocation.

2. create can take a delegate disposable, insteaf of delegating a per instance disposable, the disposal logic can be added to the delegate as teardown logic

Observable.create can take a cb that returns teardown logic instead of a disposable. createWithObserver can take a function that returns unit.