Added support for HMAC-SHA256 and other SHA* algorithms 

I've added a wrapper on top of openssl libraries to bring support in ngx lua for HMAC-SHA* algorithms.
The supported algorithms are: sha1, sha224, sha256, sha384 and sha512.

HMAC-SHA256 is especially important for AWS V4 auth.
This creates a new context for every MAC which can get expensive if you're creating / verifying many MAC values. Also, it doesn't allow for a single MAC to be updated at a later time (e.g. for large values / stream values).