Favicon changes

I put off getting the disk cache stuff done here, but this provides a base for it. There are  still a lot of pieces here:

1.) An LRUCache extending a generic cache structure (to make it easier to also put a disk cache in place)
2.) An async-image loader using NSOperation and NSOperationQueue. We need tasks that can be pushed onto the queue and cancelled here. We could write our own wrapper over GCD, but that's exactly what NSOperation does, so I figured we should use it.
3.) A FaviconFetcher that downloads pages and pulls favicons from them. This depends on a 3rd party HTML parser that basically wraps libxml.
4.) Tests for everything.