Adds timing support to announcements

announcements can be shown and/or hidden at specific times
Adds timing checks to helper
Fixes the error when no announcements are in the database
Cleanup
Merge branch 'fix' into timing

Conflicts:
	spec/announcements/announcements_helper_spec.rb
Filter new and old announcements in database
Amended gemspec dependency on rails from ~> 3.1 to >= 3.1 as stated in the README