OpenSSL BN math

Needed to fully populate e.g. backend RSA contexts from an
RSAPrivateKey.
Merge pull request #547 from public/bn-maths

OpenSSL BN math
Refer to the `d` param of RSA as `private_exponent`.
Use python.org instead of peps.io
Merge pull request #551 from public/pep8-link

Use python.org instead of peps.io
Merge branch 'master' into private-exponent
Merge pull request #550 from alex/private-exponent

Refer to the `d` param of RSA as `private_exponent`.
Use the builtin role to link to peps
Merge pull request #552 from alex/rst-syntax

Use the builtin role to link to peps