Select all checkboxes from header

Was tricky to get this feature implemented. Not as clean as I would like. Had to resort to sending events from window. Feel free to provide any suggestions though. 

Changes to 'selectionTarget' from header select/deselect all checkbox doesn't get picked up by Angular until the next digest cycle. Anyway to resolve this within React?


Select/Deselect all checkboxs header toggle enabled by `batchToggle` option. E.g., 
```
columnDefs: [
    new ngReactGridCheckbox($scope.selections, {batchToggle: true}),
    ...
```