Prepend url, too, in feed.xml.

Per the convo on 2dec333910b06f8c3ca3916a0dea833a14463000:

@albertogg:
> @parkr what about all the variables {{ site.baseurl }} variables used in the feed.xml, _includes/head.html _incluedes/header? do you suggest changing them?

@parkr:
> I would prefer not to use them in a perfect world, but this works.

@albertogg:
> But they are being used in all those files I stated.

> [...]

@troyswanson:
> Those ought to be absolute URLs, no?

@parkr:
> Yeah.

NOW THEY ARE!!!!!!!!!!

![screen shot 2014-05-07 at 12 09 21 am](https://cloud.githubusercontent.com/assets/237985/2898440/9a56d36a-d59d-11e3-8b1d-5fa98680263f.png)