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 mean something like this:

    x = 0
    while True:
        filename_part = extra.get('filename*%d*' % x)
        if filename_part is None:
            break
        # append continuation
        x += 1

I don't understand what you mean with "consistent". If you mean that clients may skip certain values of ``x`` (like ``filename*0*``, ``filename*1*``, ``filename*2*``, ``filename*3*``, ``filename*5*``), we also could extract all keys starting with ``"filename"`` from the ``extra`` dict in a linear scan.