Issue #377: MetroStyled DatePicker

A MetroDatePicker Style. Think I got it pretty close the MetroTextBox.

![image](https://f.cloud.github.com/assets/764517/283767/5f11e860-91bc-11e2-8abc-dcbac9f71bc2.png)

**Known issues/limitations**

*Watermark* 
The internal DatePicker watermark isn't very flexible. Should be possible to hide the internal one if a TextBoxHelper.Watermark is applied.

*Calendar*
The popup calendar is not styled at all.