Guide.ShowKeyboardInput for WinRT

I've implemented the Guide.ShowKeyboardInput for WinRT. It uses a XAML
to draw a PopUp with an InputBox (or PasswordBox) and two buttons, one
for OK and other for Cancel. It is similar to the MessageDialog.
Merge branch 'develop' of https://github.com/mono/MonoGame into winrtGuideShowKeyboardInput
Now compiling again on WP8 branch.
Merge branch 'develop' of https://github.com/mono/MonoGame into winrtGuideShowKeyboardInput

Conflicts:
	MonoGame.Framework/MonoGame.Framework.Windows8.csproj
	MonoGame.Framework/Windows/GamerServices/Guide.cs

Merged with new Protobuild process.
Merge branch 'develop' of https://github.com/mono/MonoGame into winrtGuideShowKeyboardInput
Throws NotImplementedException on non-XAML builds
Merge pull request #2120 from azchohfi/winrtGuideShowKeyboardInput

Guide.ShowKeyboardInput for WinRT