Add ProtocolUser for custom system plugins.
Merge branch 'develop' into ForUser

Conflicts:
	plugin/protocols/platform/android/PluginFactory.cpp
	plugin/protocols/proj.android/jni/Android.mk
Merge branch 'develop' into ForUser
issue #2317, Adjust ProtocolUser for actual situation.
issue #2317, Add plugin qh360 .
closed #2317, Add sample project HelloUser.
Merge branch 'develop' into ForUser
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into develop
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into develop
Modify the min sdk version of android platform required in plugin samples.
Modify the target version of plugin samples to android-10.
Merge pull request #3000 from natural-law/develop

Add ProtocolUser for user system.