Allow to override the options during the takeShot callback

[NBZ4live]

Sometimes it is useful to override some of the options per page basis.

This pull request makes it possible by allowing to override them when calling it as follows:

window.callPhantom({
    event: 'takeShot',
    options: {
        shotSize: {
            width: document.getElementsByTagName('html')[0].offsetWidth,
            height: document.getElementsByTagName('html')[0].offsetHeight
        }
    }
});