Add hmac-sha2-512 MAC support

This small patch adds hmac-sha2-512 support to paramiko, like #365.

Some security standards now recommend to disable MD5 and SHA1, and use SHA2 instead. This change was tested using SHA2 against RHEL6's OpenSSH v5.3p1 and Solaris 11. And was also tested with RHEL5's OpenSSH 4.3p2 which doesn't include SHA2.