Client-side page callback support

Add possibility to register functions into the phantomjs page object which are fully evaluated client-side. This can be used to remove asynchronousness from phantom callbacks such as page.onCallback(), since the evaluation takes place synchronously inside phantom instead of via socket.io in node.
Add page.onCallback()
Merge branch 'client-side-fns'
Merge branch 'oncallback'
Adding ability to specify phantomjs startup options
Merge pull request #15 from DelvarWorld/options

Adding ability to specify phantomjs startup options