implement Tor supports through a simple SOCKS5 client implemented with
pysocks. --tor is basically a shortcut for --socks localhost:9050.

we deliberately do not use the --use-tor flag of GPG 2.1 because we
support legacy but also because supporting arbitrary SOCKS servers
seems useful (e.g. SSH is a SOCKS proxy as well).

there are no explicit tests for this, we delegate responsability for
SOCKS tests to the pysocks library. integration errors would be our
own, however, but i've tested this minimally and it seems to work,
provided that the remote SMTP server doesn't blacklist Tor exit nodes
(which is somewhat unlikely) or that the user knows a hidden service
that his provider offers on the Tor network.

Closes: #53