Navbarimg rssfix

This pull request is a superset of the previous pull request (contains the stuff in the previous pull request AND a few other things). This pull request introduces:

1. Links to categories in navigation bar (same as previous pull request)
2. Possibility to add a sidebar image. README.rst modified with instructions
3. Currently, if FEED_RSS as well as FEED_ATOM are both not set, the navigation bar still shows an 'RSS' link to the right. I modified it so that the RSS link is displayed only when the FEED_* variables have a value.