Adds infrastructure code for HMAC #25.

Adds command line interface to route code to HMAC.
Adds HMAC mac mode encoding structure.
Adds perform_hmac function
Adds serialize_hmac_mode function.
Merge branch 'hmac' into next

Adds skeleton for hmac command.
Adds HMAC command. Resolves #25.
Adds documentation for HMAC command. #25.
Updates tests for new features.
Updates online help.
Bumps version.
Updates NEWS.
Updates ChangeLog.