Add TimeSpan to KeyTime converter

Just added a new converter to convert a TimeSpan into a KeyTime to allow using it to bind the KeyTime property of a animation key frame:

```xml
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="MyTextBlock"
							   Storyboard.TargetProperty="Opacity">

	<DiscreteDoubleKeyFrame Value="1"
							KeyTime="{Binding Path=TimeToBind, Converter={converters:TimeSpanToKeyTimeConverter}}" />

</DoubleAnimationUsingKeyFrames>
```