New Performance indicator

Failure moving average. Each failed verb computes as 100 and each passed one as 0. We compute a new failureIndex as a moving average of this values. This index follows closely the failures (two verbs with the same number of failures but with different distribution in time, have different failureIndex, but the same failureRatio)
Merge branch 'master' of https://github.com/rbarbera/irregularverbs
Merge remote-tracking branch 'upstream/master'
UI Style applied to Test Mode

Also, some changes into Home Screen to suppress the duplicated NavigationController
New TestSelectorView

Now it's a simple NSViewController without a table, so we can forgot the nasty scrollbar. It still builds up dynamically, so if we increase the number of test, we must figure out how to show the new data
More sensible Default images
Merge branch 'master' of git://github.com/rbarbera/irregularverbs

Conflicts:
	IrregularVerbs.xcodeproj/project.pbxproj
	IrregularVerbs/HomeViewController.xib
	IrregularVerbs/TestCardsStackViewController.m