Solver a bug that happens the first time the app is launched without a list of verbs in documents
Up/Down gestures only actives while the test is running
Merge remote-tracking branch 'upstream/master'

Conflicts:
	IrregularVerbs/CardViewController.m
	IrregularVerbs/CardsStackViewController.m
	IrregularVerbs/VerbsStore.m
Merge remote-tracking branch 'upstream/master'

Conflicts:
	IrregularVerbs/VerbsStore.m
Merge remote-tracking branch 'upstream/master'
Experimental use of frequency of use in english for verbs to assign a meaningful way to select the "level"
Documentation and preferences screenshot
rbarbera frequency model ported to VerbsStore