Background image functionality

Added code so you can set an image as background for all pages.
Use it like this: 

ThemeManager.SetBackground(new Uri("Images/AppBackground.jpg", UriKind.Relative));
Merge branch 'master' of https://github.com/rowdynl/wp-thememanager into rowdynl-master
v1.5 adds background image and brush support. Thanks @rowdynl.