Add secret key encryption via Salsa20 + Poly1305

* Adds nacl.secret.SecretBox which represents a secret key.
* nacl.secret.SecretBox().encrypt can be used to encrypt a message
* nacl.secret.Secretbox().decrypt can be used to decrypt a message
Test that nacl.secret.SecretBox works properly
Document nacl.secret.SecretBox
Merge pull request #10 from dstufft/secret-key-encryption

Add nacl.secret.SecretBox - Secret Key Encryption for PyNaCl