Add more usage examples

[atis2345]

I'm new for PhantomCSS and don't know how to use it.
I want to create page screenshoot for 1920x1080. How I can do this?

[jamescryer]

I'm happy to accept Pull Requests for more examples.

Everything you need will be in the CasperJS docs. Take a look at http://docs.casperjs.org/en/latest/modules/casper.html#viewport