DataGrid: introduce MetroDataGridCheckBox and fix GridLines Brushes

- [x] add MetroDataGridCheckBox
```xml
<DataGridCheckBoxColumn ElementStyle="{DynamicResource MetroDataGridCheckBox}"
                        Header="IsSelected"
                        Binding="{Binding RelativeSource={RelativeSource AncestorType=DataGridRow}, Path=IsSelected, Mode=OneWay}" />
```

![2014-03-24_13h13_13](https://f.cloud.github.com/assets/658431/2498916/b0506dc4-b34d-11e3-9619-198bc4dc845e.png)

- [x] fix GridLine Brushes

Closes #1157 DataGridCheckBoxColumn does not follow MA.M styles
Closes #943 Fix GridLines Color