Place HMAC classes under the HMAC module

The renames the following:

Crypto::Auth::HmacSha256 -> Crypto::HMAC::SHA256
Crypto::Auth::HmacSha512256 -> Crypto::HMAC::SHA512256

I think these class names are less cumbersome and more appealing
visually.
Merge pull request #38 from cryptosphere/rename-hmac-classes

Place HMAC classes under the HMAC module