Initial Link header extension, hrWebLink, hrLinkCollection.

* Leverage nice http-link NPM module.
* Conditionally use webpack just for angular-hy-res-link-header.
* Tweak karma runner to handle webpack usage.
* Tests for Link header feature.
* Make links first class feature with hrWebLink and
  hrWebLinkFactory classes.
* hrLinkCollection encapsulates ability to follow a
  collection of links.