Allow setting IMAP servers' SSL version

We now allow setting the SSL version used when connecting to IMAPS servers, and
do so via the `ssl_version` configuration option. We default to the current
practice (letting python's "ssl" library automatically detect the correct
version). There are however rare cases where one must specify the version to
use.

Signed-off-by: Ryan Kavanagh <rak@debian.org>
Merge pull request #42 from ryanakca/next

Allow setting IMAP servers' SSL version