py37 support, aiohttp related fixes and fix before_response_record mutability

• Support python 3.7 (fix httplib2 and urllib2, thanks @felixonmars)
• [#356] Fixes before_record_response so the original response isn't changed (thanks @kgraves)
• Fix requests stub when using proxy (thanks @samuelfekete @daneoshiga)
• (only for aiohttp stub) Drop support to python 3.4 asyncio.coroutine (aiohttp doesn't support python it anymore)
• Fix aiohttp stub to work with aiohttp client (thanks @stj)
• Fix aiohttp stub to accept content type passed
• Improve docs (thanks @adamchainz)