Added crypto_secretbox support.
* src/nacl_nif.erl: Added test for authentication failure.
* src/nacl_nif.erl: Fixed semantic bug in test variables names.

secretbox/2 returns a tuple like {enc, Nonce, CiperText} but I had
accidentally put it backwards which caused some issues.
Merge pull request #1 from tel/master

Added crypto_secretbox support