Add ability to use a remote selenium server that requires auth

Currently nightwatch seems to only run locally. Would be great to e.g. run tests on Sauce Labs using `nightwatch saucelabs`:

``` json
{
  ...        
  "test_settings" : {
    "default" : {
      ...
    },
    "saucelabs" : {
      "selenium_host": "ondemand.saucelabs.com",
      "selenium_port": 80,
      "username": ...,
      "key": ...
    }
  }        
}
```
