Expose traceur.get

I'm writing a NodeJS project which is doing custom transformations on Traceur parse trees.

I don't want to write in ES6, so if I want access to various exports, it can be tricky.

This PR exposes `require('traceur').get('codegeneration/ModuleTransformer')` etc to allow easy loading of the various modules.

Happy to consider alternative APIs but this seemed the simplest to me.