Mark mime::header::match() as const

This appears to have just been an oversight.
Decode and convert headers

It turns out that some people send me mail that's got headers that are
encoded and/or in non UTF-8 charsets.  This patch decodes them, but I
haven't actually read any RFCs so I don't know if I'm doing it
correctly or not.
Modify the "unread" column on "show"

The general idea here is that I want to try and push the unread status
of messages up to the server.  This commit simply stores the unread
status to the database, it doesn't actually do anything on the server
yet.
Sync the read flag to the IMAP server

Now when I read a message it'll get reflected in GMail!  This is
really great because it makes the notifications on my phone a whole
lot nicer: now they'll show the latest message as opposed to just the
first message in the thread that I haven't happened to click on yet.