Fixed ngReactGridCheckbox column width

[ewu02]

Default 10% cellWidth only applies when grid.horizontalScroll is enabled. This resolves several width and display issues.

ngReactGridCheckbox width can now be overwritten. Defaults to '2%', which is suitable for most cases.

e.g.,

new ngReactGridCheckbox($scope.selections, 
    {batchToggle: true,
     width: '10%',
     disableCheckboxField: 'disableCheckbox'});