Removed preference "show future releases", as
a) It was never implemented as "show" (it controlls only which releases
are fetched from musicbrainz) and
b) it is rather useless in terms of the purpose of nusic (to notifiy
about new releases)
Implemented generic mechanism that can react to new application versions AbstractApplication.onUpgrade() which is similar to SQLiteOpenHelper.onUpgrade()).

Implemented NusicApplication.onUpgrade() that takes care of scheduling the ReleaseTodayService and cleanes up the "show future references" preference removed in #648bd11.
Merge branch 'develop' of https://github.com/schnatterer/nusic into develop

Conflicts:
	TODO.md