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
Hm.  Since this is novel pyOpenSSL behavior it seems like it should be considered carefully (and documented).  Also I wonder if formatting the subject hash into a string is the most useful behavior.  It's useful for a human trying to figure out what's going on.  It's annoying to deal with programmatically though.  Maybe this should be an object with an attribute instead.