Use system feedparser if available

The reason for this is to take advantage of the system library, so that in the event of security vulnerabilities, only one package requires updating.

Applied this patch, built a new EPEL 6 package. Installed it cleanly on a RHEL 6.3 system, along with Review Board. Ran rb-site to create a new Review Board instance, loaded it up, added a git repository, uploaded a new diff.

In the EPEL build, I also explicitly removed the bundled feedparser.py from the package in order to avoid falling back to it.

Nothing seemed impacted by this, and I didn't see any errors around failure to load the library.