Config option to allow SlickGrid style scrolling performance

Added an config option similar to [SlickGrid's](https://github.com/mleibman/SlickGrid), which allows a trade off between: 
1. keeping the contents of this grid up to date at the expense of a laggy scroll button
..* by calling $digest() in the viewport directive continuously during scrolling
2. OR sacrificing continuous refreshing of the grid to keep the scroll bar responsive
..* achieved with a short timeout before rendering, inspired by the SlickGrid codebase

The new config option `forceSyncScrolling` defaults to `true` to keep the current behaviour of the ng-grid.