issue #5032:A potential crash bug of Lua binding
Merge branch 'v3' of https://github.com/cocos2d/cocos2d-x into v3_lua_fix
Export the `2d` include directory of engine on android.
Merge pull request #6596 from natural-law/v3

Export the `2d` include directory of engine on android.
download-deps.py will unzip the download zip file.
Output more detail informations
Delete external submodule, please use 'download-deps.py' instead.
Re-adds box2d and chipmuck sources, we need to use prebuilt libraries for them too.
Adds external/version.txt
Updates .gitignore.
updates travis-script.
add some comment for dependences module in Android.mk to make it simple to use other module
:= -> +=
Merge pull request #6598 from minggo/android-makefile

add some comment for dependences module in Android.mk to make it simple ...
Updates .gitignore, ignore version.json
Updates travis script
download-deps.py supports to check 'external/version.json'
Removed external submodule in .gitmodules
Adds external/config.json for download-deps.py
download-deps.py reads configuration from 'external/config.json'
Delete external/version.txt, use external/version.json instead