Add popToRootSceneWithTransition: to CCDirector.

Adds a version of popToRootScene that takes a CCTransition object, to match the WithTransition: versions of popScene, presentScene, pushScene and replaceScene.
Add popToRootSceneWithTransition: to CCDirector.
Merge pull request #671 from sdrew/develop

Add popToRootSceneWithTransition: to CCDirector.