Support for an array of reminder days

`days_until_reminder` can now be an list of days to remind after (just a single number is still supported) in case you need more then one reminder to take care of an email.
Stop using the Mail gem in favor of dealing with raw message data with the hope that this will result in better multipart message handling (so far, yes).

Getting the headers I need by hand, and manually encoding the strings I'm adding to the message all while trying to handle all the variations that mail clients send with.
Updated `days_until_reminder` help text to include information about the new list option.