Switch to using NaCl

[awnumar]

Scrap all the current encryption code and switch to using NaCl's SecretBox for symmetric crypto. I'll list a few reasons below:

• Don't roll your own crypto
• It handles authentication so you won't have to scrypt a 512 bit output and split it for AES and HMAC
• It handles padding
• It handles nonces
• Other shit

tl;dr use NaCl