Persist cookies across redirects

addresses #60

this is work in progress. **do not merge yet!**

1. ~~we should wait for https://github.com/lostisland/faraday_middleware/pull/97 to be merged, so we can specify the new released version in gemspec and remove the github line from Gemfie~~
2. i don't think there is any way to test behavior when sending a cookie using fakeweb. maybe i'll use another web mocking system just for the cookie tests