Convert MBCommon to ARC.
Remove unnecessary synthesize statements.
Test for nil in MBJSONDataFromObject.
Update README for ARC and JSON.
Remove MBJSON.

MBJSON is no longer useful as MBCommon requires at least iOS 5 or
Mac OS 10.7.
Remove MBError and MBLocalization, which are no longer used.
Remove README section about MBLocalization.