Replace EXTERNAL_HOST_NAME_WITHOUT_PORT with request.get_host()

Replacing EXTERNAL_HOST_NAME_WITHOUT_PORT with request.get_host() simplifies request_stream results.

Localhost: <http://localhost:8888/api/1/?operation=request_stream&project_id=1&session_id=1> responds with:

    {
        "stream_url": "http://localhost:8000/stream16730.mp3"
    }

127.0.0.1: http://127.0.0.1:8888/api/1/?operation=request_stream&project_id=1&session_id=1 responds with:

    {
        "stream_url": "http://127.0.0.1:8000/stream16730.mp3"
    }

Some other minor changes included, such as a README.md fix (oops) and removing API_URL from common.py too.