Emscriptenate

Patch to add Emscripten support. Basic changes:
- Add pre-build freetype, libpng, etc for Emscripten
- Add submodule for Emscripten
- Add platform directory for Emscripten
- Add code to push vertex data to server-side buffers as WebGL does not support client-side buffers.
- Lots of little fixes.

Current status is that it can run the TestCpp sample. Almost all tests run, still some visual glitches, but probably good enough to start building on top of now.

To build, run

    make -f Makefile.emscripten

JS, HTML, data files will be put into the standard proj.emscripten/bin/release directories. Note that these files need to be hosted on a webserver to run, as they depend on XHRs.

Ricardo Quesada has taken a look at the diff, and we agree that this is a great facility to add to Cocos2D-X. He's suggested using object composition for the vertex buffer code. Let's discuss on the mailing list, and if we need to re-work it, I'll submit another pull. I think this follows the structure of the current code though, so would like to start with this.