Allow using a custom requests Session instance.

This is a simple way to allow for a lot of customization, like the user agent to use when fetching the robots file, without having to go the route of managing the cache manually.

Another thing I find stands in the way of reppy being a perfect drop-in-and-use-as-is API is that I have to pass the user agent into allowed() every time; it might be worth considering if it couldn't automatically use ``self.session.headers['user-agent']``.