Add support for two-key Triple-DES

I am currently trying to unbundle libtomcrypt 1.16 from the python-crypto (pycrypto) package in Fedora Linux. The pycrypto code has long had one additional feature that is not in the upstream libtomcrypt code, namely support for two-key Triple-DES, with 16-byte keys rather than 24-byte ones.

With the inclusion of this feature from pycrypto, I would be able to unbundle libtomcrypt from pycrypto and use the system version (currently 1.17) instead.

Please consider merging this well-tested (in pycrypto) feature.