add `get_digest` method to `X509Req`

(depends on bwhmather/cryptography@1b55947c).
Writing a simple ca and need to be able verify that a certificate request received by the server is the same as the one sent by the client.  Comparing digests seems like the obvious way to do it.
Fixed.  Admittedly whole function was copied and pasted from `X509.digest`.
The test for `X509.digest` is currently failing (https://github.com/pyca/pyopenssl/issues/150) so I haven't made the corresponding change there.
Also added bit to the documentation.