Move multipart MIME parser into utils

Over on the EC branch, we need to be able to parse multipart MIME
documents in the object server. The formpost middleware has a
perfectly good MIME parser, but it seems sort of awful to import
things from formpost in swift/obj/server.py, so I pulled it out into
common.utils.

Change-Id: Ieb4c05d02d8e4ef51a3a11d26c503786b1897f60
Merge "Move multipart MIME parser into utils"