Adds a possible fix for not being able to Unbind cross-context injection bindings (see issue #87)

I was unable to remove a cross context injection binding via the "Unbind" function. I overrode 'Unbind(object key, object name)' in CrossContextInjectionBinder to check if a binding is crossContext and directly remove it from the CrossContextBinder.

I don't know how much of a hack this is into the overall scheme, but it works for me! Let me know if there are any issues with the approach I took.