Allow compiling modules to RequireJS syntax.

This adds:
  - RequireJsTransformer, which produces RequireJS syntax code,
  - src/node/to-requirejs-compiler.js, which compiles files one-by-one.

The unit tests are pretty lame, but I wanted to at least get it set up so that it's easy to add more
unit tests in the future.
Add integration tests for RequireJS.
Add make targets for running a subset of tests (unit/features/requirejs)

make test/unit
make test/features
make test/requirejs
Add Node.js API.

This API is what you get when you `require('traceur')`.
It is supposed to be used by custom Node.js scripts or tools such as Grunt or Karma.