Use SystemJS baseURL instead of relying on proxy configuration

Recently discovered SystemJS's `baseURL` configuration option which allows us to specify a path to prepend to all SystemJS load requests.

We can use this instead of having the consumer specify a Karma proxy configuration, making it a lot cleaner!

## Testing

* Ensure unit tests pass (`gulp test`)
* Try in a project that uses karma-jspm via npm linking

@trentgrover-wf