Add theming, with a Dark and Light theme

 - The light theme remains the default, and should look identical to before
 - Modify the layouts to reference colors in the theme
 - Add an option to change the theme in the preferences (note that this does
   not take effect in already running activities)
 - Modifiy the ConversationActivity as the secure drawable for the action bar
   is not handled in the layout
 - Modify ConversationFragment as the text color is not handled in the layout
 - Add calls to setTheme in the activities