Parser handles multi-part messages with attachments

Fixes #2

The multipart regexp now looks for `mixed` and `alternative` mime subtypes, and makes the boundary quoting optional (as it seems to be).

The `doParseBoundary` checks if the Part it's found itself is multipart, and if so recurses down on that Part and puts all those subparts into the flat array that's returned.  That seems like the right thing to do?

And a couple of very minor tweaks to the README.