Adding dependencies (obelisk & xdg-open)

I tinkered with these over the weekend, and I think this is going to work for now. xdg-utils is a package that contains xdg-open, which is used to launch firefox on the host. It's a big package for a little job, but it does meet the dependency left by darkmarket.

The python-obelisk also throws an error, and this solves it by building it from the git repo.