Add support for HMAC-SHA2-256 and HMAC-SHA2-512

This adds support for the new MAC modes using SHA-2 family of hashes as
set forth in RFC 6668.

See http://tools.ietf.org/html/rfc6668 for more information.

Change-Id: I09dfe44efe230021a77d81546fccc6b124c958d3
Add diffie-hellman-group-exchange-sha256 support

Support exchanging groups using SHA-256 as specified in RFC 4419

For more information, see https://tools.ietf.org/html/rfc4419

Change-Id: Iee5d29e7113a05cad4714a61321bf86b016624b8
Fix exception message in ECDSASHA2Verify

Change-Id: Ib69c29588acb60b94cf710244f4c4dc76534412f