add mark all as not watched menu item to EpisodesListFragment
add mark all watched/not watched menu items to SeasonsListFragment

which will mark all episodes of all seasons as watched or not watched,
rather than episodes in a single season.
uncapitalise menu item strings
use a service to add new shows

Creates AddShowService, an IntentService which fetches show and episode
information from tvdb and adds it to the database.

This replaces (and most of the code is directly copied from) the
old AsyncTask which did the same job