Update expected parser classes in docs

Looks like these changed since the documentation was written.
Further improve docs for parser classes
Added content to itunes_rss_item.

I've come across content:encoded on some podcast feeds i.e.
http://feeds.5by5.tv/b2w
Add to specs for iTunes Item

closes #198
Bump version for 0.6.0
Update CHANGELOG for 0.6.0