reveal used macs only once

without this, messages get larger the longer an otr session stays
active. this leads to anti-spam measures being triggered (on ICQ for
example) and is also the cause for the infamous bug mmb/weechat-otr#17
Merge pull request #49 from tribut/reveal-once

reveal used mac keys only once
Make sure AES key is always 16 bytes.

Once in a while the key was less than 16 bytes due to leading zeros
causing pycrypto to raise an exception.

See https://bugs.launchpad.net/pycrypto/+bug/1276431

Bug identified and fix suggested by Legrandin.
Merge pull request #52 from mmb/not_enough_bits

Make sure AES key is always 16 bytes.