Added support for theme in config.xml
Implements issue # blackberry/BB10-Webworks-Packager#206

Set theme for the application controls base one of the values: 'dark', 'default', 'bright', 'inherit'.
If no value set or the value spelled wrong, 'default' would be used.

Reviewed by: James Keshavarzi <jkeshavarzi@rim.com>
Tested by: Bryan Higgins <bhiggins@rim.com>