add script to produce benchmark/perf data

[warner]

In README.md, I have a section named "Speed" that provides timing data for creating/signing/verifying with various curves. I should dig up the script I used to produce that and include a copy in the source tree, so setup.py bench will produce the same table on your local machine.

Also, we should add similar data for the recently-added secp256k1 curve.

[warner]

The main script is done: run setup.py speed to print the table. Remaining tasks: add a C program to generate the corresponding Crypto++ benchmark for comparison, and update all the numbers to my current-generation computer.

[tomato42]

I'm not sure if crypto++ implementation is necessary for comparing performance... openssl speed gives performance numbers of ecdsa too.