Added serializer plugins

In the main package XML and JSON serializers using the runtime
serializers. As plugins the 3rd parties:

Json.NET (portable)
ServiceStack V3 (NET45, iOS, Android & WP8 versions) - only JSON
serializers for now
ProtoBuf.Net (portable)
Merge branch 'master' of https://github.com/XForms/XForms-Toolkit

Conflicts:
	src/XForms.Toolkit/XForms.Toolkit.WP/XForms.Toolkit.WP.csproj