Separate RSS parser from aggregator

The aggregator module doesn't do an especially good job of separating the steps of parsing an RSS feed. This first cut at separating the functionality is mostly moving code around. The actual parsing should move into an included file which can be used by contributed modules such as Node Aggregator. This improves speed slightly because RSS parsing hapens on a small fraction of page views and reduces the code to be parsed. As this new inc file matures more things such as conditional http gets (looks to already be in aggregator) and data cleaning can move out of the Aggregator and Node Aggregator modules.

Coming with this patch is better formatting for comments and more in code documentation.

I think this patch should go before 4.5. While it is simply moving a lot of code around, I believe it already is a great improvement and good stopping point for now. I have tested parsing feeds via cron.php and manual updates.