Add an appendix for experimental Gradle & Android Studio

Integrating Gradle build in cocos2d-x was discussed here: http://discuss.cocos2d-x.org/t/adding-gradle-support/17289
Conclusion is: it's not the right time yet. In the meantime, willing cocos2d-x users should be able to use Gradle if they want to.

The instructions were tested on Mac, but not on Windows and Linux.

I added the full wrapper to the repo because I couldn't find a place to download it directly (instead of having to install Gradle and generating the wrapper, as explained here: http://www.gradle.org/docs/current/userguide/gradle_wrapper.html)