mdoc format

I rewrote the existing man pages in the mdoc(7) format. Futher more, I renamed the uzbl(1) man page to uzbl-core(1) since a command named uzbl doesn't exist.

This man pages should be extended until the next release and in addition to that it would probably be a good idea to start migrating some information from the README to dedicated man pages (such as uzbl_config(5)). However, this might be a first step in the right direction.