ssl_support: Adjust to tunneling changes in Python 2.7.7 and 3.4.1.

The fix for https://bugs.python.org/issue7776 changed httplib.HTTPConnection's
handling of tunneling: `host' now points to the proxy host, so we have to
adjust the code to perform the certificate validation on `_tunnel_host' instead
when it is available.
Merged in rakuco/setuptools (pull request #71)

ssl_support: Adjust to tunneling changes in Python 2.7.7 and 3.4.1.
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