Add plugin twitter for iOS.
Merge branch 'master' of https://github.com/cocos2d/cocos2d-x

Conflicts:
	plugin/samples/HelloSocial/proj.ios/HelloSocial.xcodeproj/project.pbxproj
Modify the license of plugin twitter.
Resolve the bug in the reflection of plugin.
Merge branch 'master' of https://github.com/cocos2d/cocos2d-x
Merge pull request #2846 from natural-law/master

Add twitter plugin for iOS