introducing OVSBridge
fallback to ovsb when no OF controller is unavailable
Merge branch 'fallback' of https://github.com/thinred/mininet into thinred-fallback

Conflicts:
	bin/mn
	mininet/node.py
Return controller correctly.
Raise exception if DefaultController cannot find a controller
Merge pull request #432 from mininet/devel/fallback

Fall back to OVSBridge if no controller is available for default switch