added phantomjs capabilities option

Hi,

I added `phantomCapabilities` option which allows to pass phantomjs capabilities as defined [here](https://github.com/detro/ghostdriver#what-extra-webdriver-capabilities-ghostdriver-offers).

For example, you can:

* disable javascript, 
* change the user-agent header sent to the server,
* add username and password used for basic authentication, 
* customize any request headers
* ...

Thanks,

Saad