Add ssl_context parameter to Connection classes

After creating a shim SSLContext class in urllib3.util.ssl_, we now want
to start accepting instances of that class in general so users can take
advantage of some features provided only on certain versions of Python.

The first step is to allow users to pass ssl_context to both
HTTPSConnection and VerifiedHTTPSConnection and then pass these down to
the shim ssl_wrap_socket function in urllib3.util.ssl_. This does not
yet account for use with PyOpenSSL.