Don't prepend '?' to wsapi.request.qs_encode output

This change makes qs_encode produce proper querystrings according to RFC
3875 but we need to take that into account when appending that to a url
(that is, we need to append "?" before the querystring ourselves).
Merge pull request #13 from ignacio/issue_13

wsapi.request.qs_encode yields invalid querystrings