Fix warnings.
Add .travis.yml

This builds all libraries and examples in both debug
and release for Linux and NaCl platforms.

Add travis build status image to README.mdown.

Also, split the linux dependency installation into
its own script: install-deps-linux.sh
Merge branch 'travis' of https://github.com/sbc100/cocos2d-x into sbc100-travis
fixed #2105: Adding Travis build support for linux and NaCl.
Merge pull request #2517 from dumganhar/iss2105-travis

fixed #2105: Adding Travis build support for linux and NaCl.