add public-key encryption support

[warner]

Some day, after ECDSA (#2,#3) and random-access AES (#18) are completed and utilized, there will be Tahoe projects which will need public-key encryption support, like RSA or El Gamal or one of those. (append-only filecaps is the most likely user). Crypto++ has these, but pycryptopp does not expose any of them yet. It would be nice to have access to one of them.