Add hmac-sha2-256 MAC support

This small patch adds hmac-sha2-256 support separately as requested in #423.

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.
Added RFC6668

Added the requested changed.
ssh-hmac-sha2-256: Remove whitespace at end of line.
Merge pull request #429 from bantu/ssh-hmac-sha2-256

SSH2: Add hmac-sha2-256 MAC support

* bantu/ssh-hmac-sha2-256:
  ssh-hmac-sha2-256: Remove whitespace at end of line.
  Added RFC6668
  Add hmac-sha2-256 MAC support
Merge branch 'master' into php5

* master:
  ssh-hmac-sha2-256: Remove whitespace at end of line.
  Added RFC6668
  Add hmac-sha2-256 MAC support