Add sha256_hash(string), sha256_hmac(string)...

 - Conveniance functions for hashing a string without the need to do
   init, update, digest

 - simplify hexdigest!()
conveniance function examples
typo
port tests from nettle/testsuite
simplify generation of [algo]_hash and [algo]_hmac
Merge pull request #26 from samoconnor/master

Convenience Functions for one-line hashes and hmacs