break out feed types; add printFeed(); namespace everything

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.