Bring IE8 support to SystemLoader

This updates with all of the changes recommended. To summarize:

* Using a string-replace grunt plugin to insert polyfills where needed for the various Object.* methods being used by esnext. Verified to work in IE9 (don't have an IE8 VM on this machine).
* Constructor now calls `super(options || {})`
* Removed unnecessary `System.constructor =` assignment
* Replaced tests and Node references to use `dist/es6-module-loader.js`
* instead. Part of #209