Add support for multipart email changes from nntp

Add support for parsing multipart email from NNTP in order to parse newly signed Changes from dak.

I've kept the non-multipart processing in order to be able to parse previous Changes.

The parsing is under the assumption that the first part will always be the body and the second the signature. I think this expectation is reasonably safe.

As usual, a regression test has been added and I've tested locally the correct processing of new Changes from news.gmane.io.

Closes #144 (closed)