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
Stephen Holsapple <notifications@github.com> writes:

>> +
>> +def verify_cert(store_ctx):
>
> Just to be clear, you're saying `verify_cert` is preferable to `verify_chain`, correct? =P Confused by the diff.
>

Yep.