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.
Yes, I think adding `update`, `final` and `reset` would cover our needs (`init` could just be done inside `new`).
We actually just published a standalone module which includes exactly this functionality: https://github.com/jkeys089/lua-resty-hmac

Had I known someone else was working on this I would have collaborated with you directly.
Perhaps we could just contribute the code we've published back into the official lua-resty-string? I'd happy to prepare a PR if you think it makes sense.