Remove account symkey

Closes #176 on merge.

When looking into 176, I realized we weren't using the account symkey for anything other than encrypting the hmac keys. We were doing this because the original spec said to, which was written before we moved to SJCL, which automatically generates a symkey when told to encrypt with a public key. In this case, there's no reason to store it.

This branch removes all references of the account symkey, and tells SJCL to encrypt hmac keys with the public key and decrypt with the private key.