Extend Loader API calls for support name manipulation

New API:
  Loader.import(name, {referrerName, address}, callback, errback).
  Loader.module(source, name, {referrerName, address}, callback, errback).
  Loader.loadAsScript(name, {referrerName, address}, callback, errback).
  Loader.script(source, name, {referrerName, address}, callback, errback).
where only the first argument is required. The triple name, referrerName, address
are passed to System.normalize().

import/module are similar to js-loaders as discussed:
https://github.com/jorendorff/js-loaders/issues/89.
js-loaders 'options' in module() where we have {referrerName, address}; we add it
to import().

loadAsScript/script are parallel but parse to Script goal. These are not part of js-loaders.

Review URL: https://github.com/google/traceur-compiler/pull/645

Closes #645.