Hacked together some multipart support

Going to investigate moving this into savon-multipart after I have a working
proof of concept here. The key change is that this uses the HTTPi support
for multipart messages to check if we need to strip apart the multipart
headers from the body. Messages that are content-type text/xml will be
appended into the body that we parse with nori, and the rest will be
available in an attachments hash.