Updated PCL projects for adding Xamarin support
Added Android platform and test assemblies.
Removed unused files
Merge branch 'develop' into feature/xamarin
Fixes to portable build and initial test code for Android
Basic Android store tests now pass
Removed unused sample code
Added initial Android doc
Fixed mobile build

Removed AsXmlDocument extension method which is not supported on WP
Added BuildXamarin target
Should these be ignored ?
Added Xamarin.Android to build pre-reqs
Merged Xamarin feature branch and fixed up unit tests