fixes to sb_xmlrpcserver.py

This patch contains several fixes to
sb_xmlrpcserver.py. First, it makes sure that the
socket being bound is reusable. Second, it changes
XMLHammie.score() so that the float score is returned
directly instead of trying to be wrapped in a Binary
object. The latter won't work since Binary's expect a
string buffer, and besides XMLRPC supports floats directly.