Factor out HTTP(S)Connection -> ConnectionCls, and cleanup.

Also removed the need to None-ify global `ssl` module in tests.
New urllib3.connection module.
Factor out HTTP(S)Connection -> ConnectionCls, and cleanup.

Also removed the need to None-ify global `ssl` module in tests.
New urllib3.connection module.
Merge branch 'factorout-connectioncls' of github.com:shazow/urllib3 into factorout-connectioncls
Merge pull request #254 from shazow/factorout-connectioncls

Factor out HTTP(S)Connection -> ConnectionCls, and cleanup.