Add WeakEventManager to fix memory leaks with commands.
Improve WeakEventManager.

Add Start/StopListening.
Add Cleanup.
Add event manager for PropertyChanged and ProeprtyChanging.
Use PropertyChang(ing|ed)EventManager
Add INotifyCollectionChanging interface.
Add CollectionChang(ing|ed)EventManager
Fix IgnoreDataMamber not valid on event.
Make ViewModelViewHost implement IReactiveObject
Make IReactiveObjectExtensions internal.
Style
Merge remote-tracking branch 'origin/rxui6-master' into weak-references

Conflicts:
	ReactiveUI.Platforms/Cocoa/ReactiveCollectionViewSource.cs
Revert "Make IReactiveObjectExtensions internal."

This reverts commit b79c3a6b6acaa87bd04f8b64f5006f711df183d3.
Merge branch 'weak-references' of https://github.com/reactiveui/ReactiveUI into weak-references
Merge pull request #547 from reactiveui/weak-references

Fix memory leaks with events.