Add OpenSSL.crypto.verify_cert method.

This change adds support for verifying a certificate or a certificate
chain. This implementation uses OpenSSL's underlying X509_STORE_CTX_*
class of functions to accomplish this.

This change also adds an intermediate signing certificate/key and a
service certificate/key signed with the intermediate signing
certificate, to make testing the OpenSSL.crypto.verify_chain method
easier to test. I figured I would add it to the top level module so
other people can use an intermediate signing certificate in their own
tests.

Issue: https://github.com/pyca/pyopenssl/issues/154
What is an `X509StoreCtx`?  Perhaps this should be `X505_STORE_CTX`?