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
Python3: __bool__, im_code, im_func
PEP-8 check all files, once in separate environment, parallel in Travis

Tox: PYTHONDONTWRITEBYTECODE = 1
python3 compatibility

- __next__ for iterator interface
- six.next() to get next item
- list(dict.keys())
- popen2.popen4 -> subprocess
- s2b -> b"..." literals
- deprecated assertEquals -> assertEqual
- hub_test test_fork using run_python
- 1L -> 1 long literal
- many PEP-8 fixes
PEP8 whitespace
PyPy 2.3 compatibility fix

In PyPy 2.3 socket.socket has no _sock property which causes eventlet
failures.
https://github.com/eventlet/eventlet/pull/97
tests: env_test: ProcessBase -> run_python

Extracted inline code to permanent files
Python3 compatibility of test files