Speed up test running by using local server

As per #47. As usual, @ozh was right (:wink:), and it turned out being better to rewrite the server from scratch in PHP than to use the existing httpbin code.

Adds the `REQUESTS_TEST_HOST`, `REQUESTS_TEST_HOST_HTTP` and `REQUESTS_TEST_HOST_HTTPS` environment variables  This allows us to hijack HTTP requests while still leaving HTTPS requests separate.