Add working sha512-256.py and sha512-224.py.

Add functioning sha512-256.py and sha512-224.py, which adhere to the
SHA-512/t specification and are effectively copies of SHA-512 (excepting
different hash values, as generated by the SHA-512/t IV Generation
algorithm). Also brushed up comments/minor formatting in sha*.py files,
and added a commented-out digest statement at the end of each file (to
underscore which hash values are included in the final digest).