Port the twilio-python library to use Requests.

Major surgery on classes and objects.

- We don't need to do Unicode encoding anymore; requests takes care of this for us. Added a test that proves this is the case.

- We don't need to do proxying anymore; requests handles this for us.

- The signature for Resources is no longer `uri, auth, timeout`. That's stupid. Instead it passes around `uri, client`. That's it.

- TwilioRestClient doesn't take a random grab bag of parameters anymore. It takes an account sid, a token, and a `Transport` class which encodes connection details (proxies, retries, timeouts, host).

- There's no reason for Resources to know HTTP implementation details. Those have been moved to the client.

- Moves TwilioRestException into the rest folder, cleans up parameter naming.

- Deletes a lot of mocks and `assert_called_with` assertions from the tests.