Add key conversion from Ed25519 (sign) keys to X25519/Curve25519 (box) keys + test

[catbref]

This is a more up-to-date pull request than PR #42 and also includes test suite and compiled nacl_factory.js.

[blavenie]

@tonyg can you merge this PR, and close my PR (https://github.com/tonyg/js-nacl/pull/42). I will be able the to send you some Libre Currency (G1) (www.duniter.fr) :) I promise ! (If you create a wallet)