Added FontManager service

Tested accuracy on WP8 to get font sizes in inches. Need verification
on Android and iOS.
Merge branch 'master' of github.com:XForms/Xamarin-Forms-Labs

Conflicts:
	samples/Xamarin.Forms.Labs.Sample/App.cs

src/Xamarin.Forms.Labs/Xamarin.Forms.Labs.Droid/Xamarin.Forms.Labs.Droid
.csproj