Updates to ExtendedLabel

This update includes some general fixes and improvements to the ExtendedLabel. The sample font has been changed to one that is a more obvious custom font and the sample page has a better variety of examples for testing.

Added FriendlyFontName attribute to allow for font substitution in the Windows Phone renderer.

Fixed a bug where the label was not rendered at all if the font was not included in the Windows Phone project.
Improved the strikethrough on Windows Phone, although this will still only work for single lines.

Found a bug on Windows Phone if the sample page content is wrapped in a scrollview. The font substitution fails to render. This has been handed to Xamarin to look at.

Comments around any renderer code that remains for the obsolete font attributes.

Added updated screenshots.