Fixed #4456: Use undefined check in cc.loader and fix comment errors
Merge pull request #1706 from pandamicro/develop-3.0

Fixed #4456: Use undefined check in cc.loader and fix comment errors
Fixed #4453: add serverImg loader to support preloading image from other server.
Merge branch 'develop-3.0' of git://github.com/cocos2d/cocos2d-html5 into Iss4453_URLSprite

Conflicts:
	CCBoot.js