Parses the reason phrase from the response

Adds support for the reason phrase from an HTTP response. [Refer to this issue in Faraday](https://github.com/lostisland/faraday/issues/418) to understand why I'm attempting to add this to Excon.

I haven't written any tests for this yet because the tests are freezing on my development machine and I haven't had a chance to debug this. It freezes when starting the bad server tests. Any pointers would be helpful!