Fixed Vimeo module.

This introduces an external dependency for lxml (http://lxml.de/). The standard
library xml.etree doesn't support invalid XML and Vimeo's source is fairly
broken. There are several third party XML parsers so I just chose my favorite.
We might have been able to get away this time by using a regexp search but that
would have introduced brittle code instead.

I've also ripped out some old safeguards that didn't make sense anymore and
put the quality auto-selection code in a separate method. As the quality setting
on Vimeo is a string (hd, sd or mobile), we can't use
svtplay_dl.utils.select_quality which just picks the highest number in a list.

Signed-off-by: Anton Eliasson <devel@antoneliasson.se>