Merge pull request #784 from reaperhulk/fix-783

Partially deopaque SSL_CTX to support a pyopenssl use case

This allows PyOpenSSL to access the "method" field of the SSL_CTX struct, which allows you to see the method of an already existing context. This is useful, for example, for debugging and testing. The relevant PyOpenSSL ticket is pyca/pyopenssl#61.

Fixes #783.