Exposing some of the requests settings

Added:
* `cert`
* `proxies`
* `http_timeout` - I saw you use timeout argument to pass it in the query string to zencoder, couldn't find what for though. checked zencoder's docs. it would be nice to use one option for both cases. 

`verify` is `True` by default. I was wrong:
https://github.com/kennethreitz/requests/blob/master/requests/sessions.py#L199

Added this info to README file.