Add WebAssembly and asm.js builds for running in web browsers. Emscripten supports both targets, and the supposedly superior WebAssembly will be enabled by default any day now, so that's definitely interesting.

This is mostly for demo builds initially, though it's certainly technically possible to run the full game in the browser as well.

(See also audiality2 #28. This needs to be dealt with before we can deploy to asm.js, though it might not be an issue with WebAssembly.)