Nitpicking.
Support aes128-gcm & aes256-gcm.

Adds AEAD support for Pssht in the form of two new algorithms:
* aes128-gcm@openssh.com
* aes256-gcm@openssh.com

Pssht uses the same rules as OpenSSH during negotiation, namely:
* these algorithms are only negotiated as cipher algorithms and
  never as MAC algorithms
* if they get selected, the MAC algorithms selection is skipped
  entirely

These rules are needed to avoid a problem in RFC 5647 where conflicting
algorithms could be negotiated for the cipher & MAC algorithms
(eg. cipher = AEAD_AES_128_GCM, but MAC = AEAD_AES_256_GCM).

Proper support for IANA-registered names is planned for later (see #16).

References:
* NIST SP-800-38D for the GCM algorithm
* bozhu/AES-GCM-Python for the pre-computation table optimization
* RFC 5116 for the AEAD specification
* RFC 5647 for the AES-GCM in SSH specification
* http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL
  for the specification of OpenSSH's extensions & behaviour

Fixes #3, #4.