Some updates to the setDate method, both to code and documentation.

Since this project requires PHP >= 5.3 anyway, the check for this version is not needed anymore in Item.php.
Updated README to new GitHub Pages domain.
Some improvements in wrapper functions (including documentation):

* setEnclosure: Expects an integer for length parameter greather than zero.
* setAuthor: email parameter is also used in RSS2 feeds.
* setId: The isPermaLink attribute can be set (RSS2 only).
setDate method supports RSS2 feeds now. New method addGenerator for some advertising.
Added much more comments to the RSS2 example and used some more method the FeedWriter offers.