#145 An alternate way to handle DP WhenAny

Why: It's possible that the AddHandler method with DP's is buggy and
it is not used very much. Replace it with a more common idiom that is
used often and more robust

How: Inject a proxy DependencyObject with a single DependencyProperty
called value between the source object and the WhenAny operation. The
proxy object implements a standard DP callback which then calls onNext
on a standard Rx Subject which in turn is hooked into the WhenAny
architecture

Does This Fix The Problem: Not sure yet as I haven't tested it against
my failing code and I can't replicate the error in the tests even though
I've made the test more robust.