mail instead of tmail in mailexceptions

Hi,

I migrated mailexceptions to require mail instead of tmail.
2 reasons:
- tmail just did not seem to work on my setup (ruby 1.9.2, sinatra, 64-bit arch)
- Mikel Lindsaar suggests this replacement  (http://tmail.rubyforge.org/)
I also fixed one failing test.

what i didn't:
- gemspec is untouched

Regards,
L. Bratke