Fixed page.includeJs()

In the current implementation of node-phantom a call to `page.includeJs()` returns immediately.
According to the [PhantomJS API] it invokes a callback, after the script has been loaded and executed.

I fixed `bridge.js` to respond in that callback.

[PhantomJS API]: https://github.com/ariya/phantomjs/wiki/API-Reference-WebPage#wiki-webpage-includeJs