A minimal set of changes to support PSR-0-style autoloading:

- rename base class to \FeedWriter\Feed and FeedItem to \FeedWriter\Item
- break types out into \FeedWriter\ATOM, \FeedWriter\RSS1, and
  \FeedWriter\RSS2
- use class constants for versions
- make generateFeed() return a string rather than doing any IO itself
- add printFeed() to do output, if needed

My rationale for the last 2 items on that list is that our web framework
expects me to pass around strings, not do direct IO. I imagine needing
to grab the output and stash it in a variable or what-have-you is
a common use case, and it felt silly to have to wrap this in an
output buffer to accomplish that.

The FeedWriter class is now abstract, which means it is not possible to directly create an instance of it. Use the new wrapper classes in FeedTypes.php instead. This also obsoletes the constructor's version parameter for end-users.