Added a study hint to verbs dataset and a test mode for homogeneous sets of verbs
Learning hints

Added a second learn table view with verbs organized by group as defined on http://www.whitesmoke.com/english-irregular-verbs
History preferred view is stored in NSUserDefaults
Removed section headers from learn with hints table

Data set flattened to show in a single section without "floating" section headers
Minor text position changes in hint view
Merge pull request #40 from rbarbera/master

New hints system support