fixed page.includeJs()

previously a call to this function returned immediately
according to the phantomjs API [1] it invokes a callback, after the script has been loaded and executed
fixed bridge to respond in that callback

[1]: https://github.com/ariya/phantomjs/wiki/API-Reference-WebPage#wiki-webpage-includeJs
Merge pull request #39 from FelixHoer/master

Fixed page.includeJs()