Add OpenSSL.crypto.verify_chain 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
It might also be nice to split this test into several smaller tests, one for each of the cases being exercised.  However, it probably makes sense to wait on that until the actual API has been decided upon since that will likely change what cases there are to exercise.