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.

I'm not sure if it is customary to provide documentation for exceptions
or not, but because this exception provides an additional meaningful
attribute on the exception base class, I figured it would be helpful to
document.

The function X509_STORE_CTX_get_current_cert seems to always return a
certificate. After reviewing upstream OpenSSL package, it seems they do
no error checking on this function either, so I think this approach
should be safe.

Worst case scenario, for a case I think is impossible, we'll get a
AttributeError or TypeError.

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.