add ssl_version unittest that works everywhere

This should fix discrepancies between OpenSSL < 1.0 and > 1.0 and also
work on Ubuntu where ssl.PROTOCOL_SSLv2 has been removed
remove superfluous request parameters
remove make_headers from pool.rst

It is already in helpers.rst wasn't built anyway.
explicitly flush all gzip data

On pypy data is buffered to long due to different garbage collection
strategies [0]. By explicitly closing the dummy files those buffers are
flushed and data is not truncated.

[0] (http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies
explicitly close listening and connected sockets
Merge pull request #147 from t-8ch/fix_pypy_tests

Fix pypy tests.
Merge pull request #142 from t-8ch/fix_ssl_test_on_old_openssl-2

Fix ssl test on old openssl.
Merge branch 'master' of https://github.com/shazow/urllib3