Changed scrypt mixer/hash settings

Salsa20/8 & sha256 are the default mixer/hash for most scrypt
implementations. It makes sense to keep this file format in line with
the de facto standards, in case any other clients want interoperability.
Added Profile class for saving tox messengers

This commit adds the Core::Profile class, which manages the
saving/loading of tox messengers. The files it creates contain an
encrypted copy of the given tox messenger, as well as some information
about the saved 'profile' (a name, saved date, and scrypt/nacl key
parameters).