Add support for encrypted data bag items v2 

Hi,

This is an updated version of my previous pull request (https://github.com/coderanger/pychef/pull/26). 

As you suggested, I got rid of m2crypto wrappers over OpenSSL functions by using them directly (via ctypes) - please see chef/aes.py file. 

Please let me know if this looks good.

Best regards,
Kamil