Updating link
Fixes issue of accent color not applied to WP8 controls

In Windows Phone 8 the accent color is used in controls for pressed and
focused states.  Due to how resources are evaluated any resource based
on accent color needs to be specifically set. Also renamed the
SetColorAndBrush() method to be more accurate.
Update README.md
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));
Forgot to commit the NuGet updates to version 1.4
v1.5 adds background image and brush support. Thanks @rowdynl.
v1.5 bug fix before nuget publish
Better support for Checkboxes: BorderBrush is now set properly for light theme
Another enhancement: when using dark theme, Checkboxes had a wrong border/background (which is fixed by this commit)
Version 2.0: Windows Phone 8 updates for checkbox, upgrading project to Visual Studio 2013. Please use version 1.5 if you need to use 2012 or ship a phone 7.1 OS verison app.