v0.10.0 - New signature API

• Signature API breaking change! nacl.sign and nacl.sign.open now deal
  with signed messages, and new nacl.sign.detached and
  nacl.sign.detached.verify are available.

  Previously, nacl.sign returned a signature, and nacl.sign.open accepted a
  message and "detached" signature. This was unlike NaCl's API, which dealt with
  signed messages (concatenation of signature and message).

  The new API is:

  nacl.sign(message, secretKey) -> signedMessage
  nacl.sign.open(signedMessage, publicKey) -> message | null
  

  Since detached signatures are common, two new API functions were introduced:

  nacl.sign.detached(message, secretKey) -> signature
  nacl.sign.detached.verify(message, signature, publicKey) -> true | false
  

  (Note that it's verify, not open, and it returns a boolean value, unlike
  open, which returns an "unsigned" message.)

• NPM package now comes without test directory to keep it small.