Remove margins from MetroFlatButton style

I don't think controls should have set a margin in their style. 
In one of my dialogs I used this code to increase the distance between the buttons:
```XML
    <StackPanel
      Margin="0 16 0 0"
      Grid.Row="1"
      Orientation="Horizontal"
      HorizontalAlignment="Left">      
      <Button 
        MinWidth="80"
        Command="{Binding OkCommand}"
        Content="OK"/>
      <Button 
        MinWidth="80"
        Margin="8 0 0 0"
        Command="{Binding CancelCommand}"
        Content="Cancel"/>
    </StackPanel>
```
and got this result: 
![image](https://f.cloud.github.com/assets/73690/1499048/8d285dea-484d-11e3-9ec0-798fc45c4bc1.png)

I'm a strong supporter of the [principle of least astonishment](http://en.wikipedia.org/wiki/Principle_of_least_astonishment), so when I try to set a horizontal margin, I was surprised that the height of the button was changed too. Instead I had to figure out the default margins and set the margin to `"8 3 3 3"` to get the correct result. :astonished: