Fix for issue #36: Remove BeautifulSoup dependency

As per subject.

Run-time behaviour should be identical. Tested with ur-sim.

Note: I opted to use lxml instead of `urdfdom_py`, as the latter has some issues parsing the UR5/10 urdfs and we only need access to a single custom element. Rosdep keys for `python-lxml` were recently added (for `urdfdom`) for all (Ubuntu) versions, so this should not be an issue.

Error handling is rather primitive right now (as in: user is immediately presented with stacktraces), but I did not want to change that in this PR.