Add a MIME parser

This (at least attempts to) parse a mail message into a MIME document
tree.  It's not very well tested, but at least it's capable of parsing
some simple documents to find the text/plain part.

As a result of prior commits, "show" now prints out the text/plain
part of messages that this supports.  There aren't any character
conversions anywhere, so it doesn't work correctly with base64 encdode
text.
Parse email address headers

This parses the email addresses headers in such a manner that you end
up with a single address per header, and that all information but the
email address is removed.  This is effectively a normalized format
that lets me more easily perform address book lookups.