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.
Thanks for the feedback. Line 84 with `C.HMAC` method creates a new context each time indeed. 
This worked perfectly fine for my use case, but I also see your use case as another nice-to-have. 
Would adding 3 new methods similar to `hmac.h` : `init`, `update` and `final` and maybe a 4th `reset` cover all scenarios ?