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!
I don't think this will work as intended, unfortunately as `"HTTP/1.1 200 Etc".to_i # => 0`. I think it might have to be something like `until line = socket.readline && status = line[9,11].to_i` and then you could read the reason stuff from the line value, perhaps with `line[12..-1]` (rather than the split which is likely slower).