Replace RMail by Mail

RMail got removed from Gentoo because it does not play well with Ruby1.9 (I just believe this fact without further checking). Therefore I replaced the mail-handling library by [Mail](https://github.com/mikel/mail) -- this also comes with the advantage that Mail does a lot more by itself and therefore reduces the mail-handling code in feed2imap.

I checked the new code with mail-2.5, as mail-2.6 is broken in Gentoo. But from the Changelog (and from the fact, that I developed against the 2.6 documentation) it should also work with mail-2.6

With commit 84d3a60 I also fix the use of HTTPFetcher -- as it turns out include_images was broken for some time without anybody noticing :)

**NB**: I'm not a Ruby programmer, so please inspect my code for it may not be in Ruby style.