Add test from bug #195, thanks @kentcb
Fix the dumbest mistake in the world. Fixes #195
Move CreateDerivedCollection and friends to their own file
Create a disposable version of ReactiveCollection and make sure the subs that CreateDerivedCollection take get disposed. Fixes #199
Add a test to verify disconnect occurs
Fix possible nullref if someone decided to pass null into the derived collection disposable
Merge pull request #200 from reactiveui/fix-derived-collection

Fix several bugs in derived collections
IReactiveAsyncCommand.RegisterAsyncObservable

Moved RegisterAsyncObservable onto the interface so
RegisterAsyncFunction and others could be moved out to a static class
and used against the IReactiveAsyncCommand interface