Restore bin/traceur.js to commits.

Our loader support is not far enough along to allow us to switch to pure node_module builds.

TBR=@arv

Review URL: https://github.com/google/traceur-compiler/pull/678

Closes #678.
Define ModuleStore and System.
Rename modules.js -> ModuleStore.js.
Former System now ModuleStore.
System now an instance of Loader.
Forward get/set/normalize through Loader/LoaderHooks/ModuleStore.