Added HMAC functions.

Fixes #20.
- Added VerifyHmacSha256 (crypto_auth_hmacsha256_verify)
- Added VerifyHmacSha512 (crypto_auth_hmacsha512_verify)
- Added SignHmacSha256 (crypto_auth_hmacsha256)
- Added SignHmacSha512 (crypto_auth_hmacsha512)

Also added some simple tests.