Several minor aesthetic changes

There is now a length limit to the userinfo span; if the username is too long, it is cut off and an ellipsis is appended to the end. This limit is shorter for the "small" version of the skin than the others.

The logo on the "medium" version of the skin is now centered over #contentwrapper and not the whole page.

Note this doesn't fix issue 11 (mobile skin not being used); I suggest we merge this before we address it.