Minor change to clarify the meaning of CardsStackViewController code in review mode
Added a statistical analysis. For each verb we will store average response time and failure ratio. We add a third view to show this statistics.
Hidden the comparation functions inside the Verb model
Merge remote-tracking branch 'upstream/master'

Conflicts:
	IrregularVerbs/VerbsStore.h
Merge pull request #28 from rbarbera/master

New statistical analisys with historical data an simplify verb interface