@never_cache decorator

I built API based on bottle,  communicate with flash client.

I found that It's more convenient  using a decorator to set no-cache than  write
`response.set_header` in all GET method functions.

So, I added the `never_cache` decorator, and tests

    from bottle import never_cache

    @app.get('PATH')
    @never_cache
    def my_func():
        return 'hello'