Allow scripting from JS

[kripken]

Compiling code in C++ that uses bullet works fine, but using the compiled library directly from JS is very hard.

1. Generating a JS API from the low-level LLVM constructs is hard.
2. Not all the necessary stuff is even compiled - looks like the compiler decides not to compile classes etc. it doesn't need. Also, we have none of the enumerations and #define constants that are useful in C++ code.
3. To use all LLVM optimizatons and DFE, we need a complete binary, not just a library, or else the optimizer will remove even more stuff we might want.

We need some new tools and ideas here. Or maybe we should just manually write the API in JS. This would give the best results, but take some initial effort.

[cjcliffe]

I'd vote for doing our own JS API -- I think it would ultimately produce a simpler and more JS-friendly interface.

[kripken]

We now have autogenerated bindings using Emscripten and CppHeaderParser.

It might make sense to add another layer on top of that for something truly JS-friendly, but I have no idea what that would look like.