Added DI interfaces for Resolver & Container

Simple Resolver interface acts as a generic resolver that can be
implemented using 3rd party DI containers. Hopefully this can somehow
be used to extend Xamarin DependencyResolver.

I will put in some 3rd party wrappers into Plugins project later on. I
have tested about 5 of them and some are PCL compatible while others
only work with WP8 and/or Android. There is a simple container
implementation as well to give a little more features than
Xamarin.Forms container.
Merge branch 'master' of https://github.com/XForms/XForms-Toolkit

Conflicts:
	samples/XForms.Toolkit.Sample/App.cs