Crypto::SecretBox - secret key crypto with NaCL

This implements bindings to the `crypto_secret_box` family of functions
in NaCl.  They offer authenticated secret, rather than shared, key
cryptography, using the same primitives (xSalsa and Poly1305) as the
public/private key crypto in Crypto::Box.

Fixes #5.

Also, this PR tidies things up with the renames we talked about, `Boxer` => `Box`, `SecretKey` => `PrivateKey`, `#unbox` => `#open`.