Spec & API update

This is quite a big update coming through - runs against https://github.com/ModuleLoader/es6-module-loader/pull/113 and Traceur master.

It is all ready to go now, as soon as the next release of Traceur is made, releases will be posted.

There are quite a few API updates in line with feedback from @justinbmeyer, comprehensive support for circular references and an update to the latest spec.

I'm in the process of updating the README and providing better documentation on the changes, which I will post here soon.