Add 'Priority' to the subreddit table.  'All' and 'Frontpage' have very high priorities so that they are pinned to the top of the list.  Closes #57

TODO:
 - Make 'Frontpage' actually open the frontpage of reddit, right now it opens the /r/frontpage subreddit.
  - Make tapping a subreddit in the list increment the 'priority' in the database.