RFC2231 filename continuations of Content-Disposition header.

This patch allows you to appropriately deal with filename continuations in Content-Disposition header.
The params continualtion is defined in RFC 2231(http://tools.ietf.org/html/rfc2231).
When specifying a long filename as a attachment file, a string of the filename will be sometimes splitted into several pieces, like 'filename*0*=' and  'filename*1*='.

Requirements: email.utils
I think we could change this into a while loop which breaks if the ``filename_part`` starts to become ``None``.