Added --url option to iptest

Allows `iptest js` to be used against remote servers.

Example usage: `iptest js --url='http://localhost:8888'`

Pinging @rgbkrk who requested this.
Another minor point: I'm pretty sure `requests.get(foo)` is the same as `requests.request('GET', foo)`, but shorter. Feel free to correct me if you know otherwise.