Creating a callback option

[wiltonlcsj]

I've inserted a option to be a callback function. I don't know if it's where must be, but worked for me. For example if a user should submit a form after the iframe has been loaded and closed (that was my problem).

[CitizenOfRome]

Thank you very much for taking the time to create this pull request.

The plugin already has the deferred option where you can pass a jQuery Deferred object with multiple callback functions attached.

And because we lack a reliable way to know if the printing failed, the Deferred object is always resolved after the actual printing has been triggered.

Hope this makes sense?

See also: #50