Augment feed data with the proper self href and any hubs the feed may use

This patch adds an "extend" parameter to the request, which when it is true, allows feediscovery to go one step further, retrieving and parsing the feed(s) discovered. Once the feeds are retrieved, we can look for rel="self" to see if it is different than that which was parsed originally from an html document, and also to extract any hubs. These 2 pieces of data allow PubSubHubhub subscriptions to be made much more efficiently.

Without extend, the code works as previously, by returning the urls found in the html page, therefore avoiding breaking any existing clients (hopefully). I still need a better library of urls for test cases and perhaps add some real tests. But, I've been running this for a few days and it's working well so far.