Dispose issue

Hi Paul,

As discussed, here is a repro and probable fix for the `ReactiveNotifyPropertyChangedMixin` dispose issue I brought up on google groups. Please note that a couple of tests (`DerivedCollectionShouldBeSorted` and `ItemsControlShouldGetADataTemplate`) were already failing right off the bat. Also, I tried to verify by building ReactiveUI.sln as per your instructions, but many platforms are unsupported on my machine - not entirely sure why.

Finally, sorry my commits aren't the cleanest. Still getting my head around git.

Best,
Kent