wsgi: capitalize_response_headers option

Setting this to False provides compatibility with broken clients
which expect response header names in particular case, such as
ETag by AWS Java SDK.
https://github.com/eventlet/eventlet/issues/80
python3 compatibility: range, zip, StringIO
python3 compatibility: print function
PEP-8 fixes, tox runs pep8 check

For now, pep8 check is only run for some files known to be clean,
we should clean the rest and enable pep8 check for all files then.
python3 compatibility

- dict.items().pop() in proc
- str/bytes in backdoor, api_test, ssl_test, test__refcount
- import httplib from six
- PEP-8 fixes
WIP