Provide 'beforeChangePage' event and proceed() to allow user-code to prevent this action

* Instantiate using 'new this.pageHandle'
* Add 'beforeChangePage' event with user-code invokable 'proceed()'
* Provide a reference to the paginator via 'PageHandle.paginator'
* Update tests

References
https://github.com/wyuenho/backgrid/issues/281