To maintain a Pythonic API, we need to initialize the store context
object at object instantiation time so that it is possible to modify the
trust store (a legitable use case) after the object is created.

As the store context implementation becomes more featureful, this will
become more important. E.g., when we add support for
`X509_STORE_CTX_get0_param` and X509_STORE_CTX_set0_param` to change
verification parameters, we'll want to do this.

This change also adds a very simple `set_store` method mostly to make
the initialization and modification changes easier to test.

Apply the changes that we've been talking about in
https://github.com/pyca/pyopenssl/pull/155 regarding the placement of
verify_cert, viz., moving verify_cert from top level of crypto into
X509StoreContext.

This makes the pyOpenSSL API slightly different than the OpenSSL API,
but the plan will be to add back a verify_cert to the top level that is
nice to use.

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

Fix a regression in Context.check_privatekey which caused it to always succeed.

Fix several typos.