allow Box.decrypt to get passed a nonce parameter

[bdargan]

Needed Box.decrypt to handle a nonce buffer.

also includes the Nan to fix #124 (comment)

[paixaop]

@bdargan thanks for the pull request. The Box.encrypt method returns a cypher box which includes the nonce already therefore no need to include it as an extra parameter.
What's the use case?

[bdargan]

Main use case is to encrypt and serialize a Box and then deserialize and decrypt it, this requires the original nonce
.
Also there is inconsistency with docs and method signature on Box.decrypt. 2nd param described as a nonce, but this is not on the method signature.

thx

[rawr51919]

@bdargan Can you please fix the conflicts?