Added a check for a restriction in the ATOM specification regarding the number of atom:link entites.
Do allow the content element in ATOM feeds only.
Die when setEncloser is used on a non-RSS 2 feed.
Built-in support for author elements in RSS2 feeds.

This was previously already possible by using the custom addElement function.
Added support for sub-elements of the author element in ATOM feeds.

It's now possible to specify an email address and an URI.
Some documentation fixes and additions for phpdoc