namespace FeedWriter

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
- updated example code

My rationale for the generateFeed()/printFeed() change 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 wrap this in an output buffer to accomplish that.

I went with FeedWriter for the base namespace, per @mibe's original suggestion, since it seemed nice and descriptive. If @ajaxray would strongly prefer that it be something like \axjaxray\FeedWriter, we could go that route easily enough.

Any comments appreciated.