add user agent css

As discussed in !304 (merged) this replaces special casing in the initialiization code with a user agent css file.

There are some tricky things:

1. presentation attributes have the same priority of "author" css, hence I needed to rework things to not "replace" properties when we get a value from the user agent css. Ideas of how to make this in a cleaner way would be very welcome :)
2. I had to comment out various rules from the user agent css documented in spec, since we do not parse them
3. I noticed that cargo is not smart enough to rebuild if ua.css changes... any idea how to achieve that?