Implement hmac-sha2-* MAC schemes and SHA2-256 DH group exchange

As mentioned in #161, this implements some of the stronger MAC schemes introduced as of [OpenSSH 5.9](http://www.openssh.com/txt/release-5.9): `hmac-sha2-512` and `hmac-sha2-256`. It also adds the `hmac-sha2-512-96` and the `hmac-sha2-256-96` schemes, which are not implemented in OpenSSH but are [defined in IETF's draft for SHA2 MACs in SSH](https://tools.ietf.org/html/draft-dbider-sha2-mac-for-ssh-04).

Additionally, it implements the Diffie-Hellman group exchange kex algorithm with SHA2-256. OpenSSH requires it to be used if `hmac-sha2-*` is used.