Support for AES-256

* put in the /crypto directory under a new package name: aesencryption
* this will use scrypt to generate keys from passphrases to be more
  secure than the initial method of iterative SHA256 hashing
* all messages will include the encrypted data, with a salt prepended
* added scrypt to GoDeps.json (revision number???)
* small test suite for the aesencryption package
* no integration tests yet