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.
@jkeys089 sounds great ! I'm all for collaboration also. 
For some reason I'm thinking that the `init` method makes sense if we want to keep the exiting `digest` method, just to keep the API simple for the cases that don't need appending capabilities.