Use HN API (and other fixes)

Probably should have made this multiple PR for the different commits, so sorry in advance.

Anyway, I noticed your endpoint wasn't working properly for a while, so I updated this to use http://hn.algolia.com (the HN API). It's not bulletproof, as it doesn't have proper URL matching, but using the URL as keyword works for 99% of things (besides, for example, root level site searches, a la http://github.com).

I also changed the background page thing into a script and made it an event page meaning it gets unloaded when not in use to save memory and removed the MD5 library which your endpoint was using which is no longer required.

This PR is probably outside the scope of your extension which was exclusively designed to use your endpoint, but I thought I'd lodge the pull request anyway in case others wanted to get this working again or you were interested in merging it in.