Refactored session management system to handle cookie and crumbs better.

Added fixes for #167, #166, #160.

yahoofinancials/etl.py

@@ -55,7 +55,7 @@ def get_data(self, session, url, request_headers=None, params=None, proxy=None,
  params=params,
  proxies=proxy,
  timeout=timeout,
- user_agent_headers=request_headers or self.request_headers
+ headers=request_headers or self.request_headers
  )
  return response