Marionette testing and network simulation

This adds Marionette testing via the `npm run-script marionette` command, and also a script to change the local network setting between the proxy and the client.
You can use it with `npm run-script networksimulation EDGE|HSDPA|LTE|default` (use `default` to restore default config).
Can we have the top 100 (or more) HTTP sites out of Alexa's top 500 instead?
The performance test can take some time, but it should give us some statistically relevant numbers.