Use external modules for the TypeScript Compiler

[basarat]

Would help pick and choose parts of the compiler. E.g. the createSourceFile function in parser.ts is pretty useful on its own. I'd rather just import parser = require('/tsc/parser') and let the module system do its resolution of all that I am using.

Probably only possible once we have multi external file to single output: #17

[RyanCavanaugh]

The compiler needs to run in as many environments as possible, including ones that don't have external module loaders (e.g. tsc).

It's much easier to create an external module out of loose code than vice versa.