add new tutorial about android，part 1. introduction.
add how to use jni.
add how to use jnihelper and screenshot.
Merge branch 'master' of https://github.com/cocos2d/cocos-docs
change folder name.
Merge branch 'master' of https://github.com/cocos2d/cocos-docs
change tutorial ’ how_to_invoke_android_api_in_cocos2dx’ and add it to catalog.
Merge pull request #60 from Ivenyang/master

fix issue #3520 tutorial ’ how_to_invoke_android_api_in_cocos2dx’ and add it to catalog
modify spell.
add sample code for chapter6.
Merge commit '7478fd8d6181491ddab9c4515acfa20d2fbeec72'
Merge pull request #61 from Ivenyang/master

fix issue #3347 modify chapter 6.
add sample code for chapter7.
Merge commit 'd405002915c145614a2518beed10f89c2e500c68'
Merge pull request #62 from Ivenyang/master

fixed #3305, add chapter7 sample code.