Support for "text/x-vcard; charset=utf-8" as a value for getcontenttype

I tried to use pycaldav with the Baïkal carddav server. Initially I couldn't get any vcard. Debugging a bit revealed Baïkal returns "text/x-vcard; charset=utf-8" as a value for getcontenttype.

This branch adds this new value to the accepted ones, it may need to be smarter though, as maybe other servers will return a slightly different type (quoting "utf-8" or something like that)