Added support for setting widget view-controller from within XML views (ALOY-545)

New pull-request based on master, as requested in:
https://github.com/appcelerator/alloy/pull/111#issuecomment-14968654

JIRA ticket ALOY-545:
https://jira.appcelerator.org/browse/ALOY-545

Blog describing purpose:
http://fokkezb.nl/320-widget-name