Updated README.
add Item&setContent() for Atom feeds

- Make it easy to get a <content> element on Atom entries
- Also spells "enclosure" correctly - "setEncloser" -> "setEnclosure"
- Fixes Atom test file on case-sensitive filesystems
- Tack a date_default_timezone_set() on tests, to avoid warnings
Implemented function to add multiple custom link elements. This fixes #10.
Updated README
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
Added documentation section
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.
Bugfix: Filter atom namespace prefixes also in feed items.
Added support for attaching media to ATOM feed items.
Added composer.json
Whitespace