Added new (de|en)cryption options

The put command now supports --encrypt=KEY and the get command now
supports --decrypt=KEY to use AES 256 in CBC mode via PyCrypt
https://www.dlitz.net/software/pycrypto/ (perhaps more options
later).

Also provided reusability of such through swiftly.dencrypt and
swiftly.filelikeiter -- the latter based on some quick work I'd done
before for OpenStack Swift.