Don't publish sending mac keys

 * Closes #42

 * The finite-state analysis of the otr v2 spec revealed an attack on
   message integrity. As a result, only mac keys used to receive
   messages can safely be published to allow forgeability of
   transcripts.

 * This change to the spec is seen in,
   http://sourceforge.net/p/otr/libotr/ci/58fd90cb77c836ff9fa762e91d2b2becc6d5aae8/