use 'cryptography' instead of 'pycryptopp'

[daira]

See ​http://www.mail-archive.com/cryptopp-users%40googlegroups.com/msg06857.html for motivation.

Zooko wrote

In the future, I intend to work toward replacing Crypto++ entirely in pycryptopp, for reasons of compilation, portability, deployment, etc.

The biggest single problem I have with Crypto++ is that it is written in C++. Every couple of years this causes a deployment headache for

me. The most recent example is that the newest and best way to interface native code to Python -- cffi (​http://cffi.readthedocs.org/en/latest/) doesn't support C++ at all. I think I'd rather have the simplicity of using cffi and give up the advantages of Crypto++. That means I have to adopt some other implementation of AES and of RSA, most likely by relying on a future release of pyOpenSSL which is itself based on cffi and which exposes the lower-level API of OpenSSL to Python land.