Added method to generate the feed in memory

I have implemented a new ```FeedWriter::generateFeedPreview()``` method. It produces the same information as ```FeedWriter::generateFeed()``` but in memory, without echoing to the browser. In this way, the generated feeds can be further manipulated before sending to the browser. Potential applications are feed caching, embedding feeds into other documents, etc. I used the new feature to integrate FeedWriter in [oowee](https://github.com/uavster/oowee), my website engine, which requires full control of what is echoed to the browser.

The new method outputs an associative array with:
- String with feed contents
- String with mime type of contents
- Array of headers that should be sent with header()