Symmetric Crypto interface of crypto-ios should be changed to use NaCl instead

As we discussed on our meeting[1], we should change the symmetric encryption interface that currently uses CommonCrypto, to use NaCI instead, introduced with our version 0.2 of the crypto-ios library.

[1] http://transcripts.jboss.org/meeting/irc.freenode.org/aerogear/2014/aerogear.2014-02-05-12.49.html