Encrypted data files, using sodium

This lets clients optionally encrypt their save files with a password. Please do not merge into master, but create a new branch. (I cannot make this request myself because Github's PR interface is limited.)

NaCl doesn't have the requisite functions, so this functionality is limited to sodium.

I also haven't tested it, which is why I'm requesting that this be a separate branch.
Don't include "../toxcore/crypto_core.h" and 
#include "crypto_pwhash_scryptsalsa208sha256/crypto_pwhash_scryptsalsa208sha256.h"
#include "crypto_pwhash_scryptsalsa208sha256/utils.h"  in a public header.

Include them instead in the .c