Add `String.prototype.repeat` polyfill

http://mths.be/repeat
Improve `String.prototype.codePointAt` polyfill

This patch adds the `CheckObjectCoercible` step that was previously missing.

Review URL: https://github.com/google/traceur-compiler/pull/560

Closes #560.
Fix `String.prototype` polyfills

Ref. #562.
Rename RequireJs and NodeJs transformers and options

The options are now cjs and amd.

The transformers are now AmdTransformer and CjsModuleTransformer.

Fixes #547
Replace Project with Loader in repl.html

Adds InterceptOutputLoaderHooks module for grabbing transcoded source and map.
Changes Loader.evalAsync() to Loader.module(source, options) closer to spec.
Adds name to Loader.eval() calls, as proposed in https://github.com/jorendorff/js-loaders/issues/82.

Review URL: https://github.com/google/traceur-compiler/pull/564

Closes #564.
Remove assertThrows and use assert.throw instead
Fix error in parsing Arguments

Make sure we do not allow trailing commas in arguments.

Fixes #570