Full Atom support

[lilyball]

FeedParser currently understands the atom:link tag, but it needs full support for parsing an entire Atom feed.

[jabley]

OK, do you have any thoughts about what you'd want that to look like? I need Atom support, and I would prefer to contribute something rather than just rolling my own.

I pulled down Sam Ruby's feed (it's likely to be a good example, have good corner cases in it like non-ASCII characters and SVG) and then created a test case which fails.

If you have ideas as to what you'd like the API to look like, please share! I don't have lots of Obj-C experience to tell me what it would look like. I am reasonably familiar with Abdera and ROME from Java, and the feedparser from Python.

[lilyball]

The API shouldn't change much at all. The only API changes should be adding accessors for values that are common in Atom feeds but aren't currently supported. All Atom work should instead be done on the back-end, the same way the current RSS support is done.