Improve crypto_util coverage

This PR includes commits from #126.
Oh, please excuse me, I haven't noticed the docstring... indeed `X509_REQ_get_extensions` is not being wrapped in https://github.com/M2Crypto/M2Crypto/blob/master/SWIG/_x509.i. It seems like using `openssl` binary is no better than grepping `X509.Request.as_text()`, but the latter doesn't feel good enough either. I'll try a bit more to find a better solution.