Boxer => Box

Also #unbox => #open.  And fixing some specs.
SecretKey => PrivateKey

This avoids confusion with SecretBox.  Though public and private don't
abbreviate quite so nicely.  Ah well, such is life.
Move the #box and #open specs to be shared.

Since they can be, as long as we use the right key for the secretbox...
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.
Merge pull request #6 from namelessjon/dev/secretbox

Crypto::SecretBox - secret key crypto with NaCL