fix today forground for calendar #683

#683 fix the today foreground for calendar if the calendar is on a grid (strange bug).
![2013-10-04 15_46_34-glowwindow](https://f.cloud.github.com/assets/3532342/1269573/724b132c-2cfb-11e3-81de-39bd0afb7b35.png)
![image](https://f.cloud.github.com/assets/658431/1270170/aedd1ab0-2d05-11e3-8cab-c4fad1b145cf.png)
```xml
<Controls:MetroWindow x:Class="MahAppsTestCSharp.MainWindow"
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:Controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
                      Title="MainWindow"
                      Height="350"
                      Width="525">

    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Colours.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.AnimatedSingleRowTabControl.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseLight.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/FlatSlider.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>

    <Controls:MetroWindow.GlowBrush>
        <SolidColorBrush Color="{DynamicResource AccentColor}" />
    </Controls:MetroWindow.GlowBrush>
    
    <Grid>

        <StackPanel>
            <Calendar />
            <DatePicker />
        </StackPanel>
        
    </Grid>
</Controls:MetroWindow>
```