make flask resource paywalled by using a decorator

[pcppcp]

It would be nice to have resource decorator that'll magically make it paywalled

i.e.

@app.route("/paywall/<id:int>")
@app.paywalled(price=1)
def paywalled_content():
    ....

I am not sure how to implement it now (it might require some refactoring), but it is a good idea to think about in a shower.

[pcppcp]

that's how it should look like

https://github.com/raiden-network/microraiden/issues/187#issuecomment-349255790