Handle feed burner xhtml atom feeds

Looks like Feedburner is now burning XHTML Atom feeds as well, so this Issue #58 started popping up again. Here's some sample feeds:

http://feeds.feedburner.com/TheBittenWord
http://feeds.feedburner.com/RotoAuthorityFantasyBaseball
http://feeds.feedburner.com/NotInventedHere

You mention that you'll be putting the ```preprocess_xml``` flag in configuration in Issue #214. A class-specific ```preprocess_xml``` flag might be a bit too specific. For instance, if you want to preprocess xml for both ```Atom``` and ```AtomFeedBurner``` then in my code I'll need to do:
```
Feedjira::Parser::Atom.preprocess_xml = true
Feedjira::Parser::AtomFeedBurner.preprocess_xml = true
```
Seems like if you want to preprocess the xml for ```Atom``` feeds, you'd probably want to do it for ```AtomFeedBurner``` feeds, so one flag would suffice.