Allow scrollRowIntoView to optionally center the row in the viewport

This is a alternative to @janek37's pull request [added scrollRowToCenter](/janek37/SlickGrid/commit/b5b99bbcb477270b866fdbc85e198de9138f601c). As mentioned [here](/mleibman/SlickGrid/pull/724#issuecomment-23353846), it adds an optional third parameter to `scrollRowIntoView` called `doCentering` which will center the row in the viewport if `true`.