Authenticated encryption with PyCrypto

[Daniel]

geremy condra wrote:
> I'd also note that you aren't supposed to use RandomPool anymore,

OK, I updated the recipe to use os.urandom()

> and that AES-192 is frequently recommended over AES-256 for
> new applications due to a number of recent developments in
> the cryptanalysis of its key schedule.

Changed easily enough. The updated recipe defaults to AES-192.

I also made one other minor tweak: the global constants were moved to
class-level so they can be overridden more easily if needed.

Daniel