Datepicker issues

working on the datepicker issues, initializde by @vegar

As discussed in issue #469, #471

- [x] Possible to change font size used in texbox
- [x] White Background for calendar on DarkTheme
- [x] Calendar button symbol adapts to textbox height

![image](https://f.cloud.github.com/assets/658431/1229730/2e4e6d3a-27c0-11e3-95b0-79e9c5a20ccb.png)
- [x] use same hover color as textbox does
- [x] Possible to change font size used in calendar
- [x] Calendar should use AccentSelectedColorBrush as foreground color for the selected date

![image](https://f.cloud.github.com/assets/658431/1231205/805cd91e-283c-11e3-84bb-db67d9fc4916.png)
![image](https://f.cloud.github.com/assets/658431/1231206/851aa17a-283c-11e3-9167-f5aa64343e57.png)
![image](https://f.cloud.github.com/assets/658431/1231208/88018c3c-283c-11e3-924c-474e7d0520a9.png)
![image](https://f.cloud.github.com/assets/658431/1231209/8a2dc674-283c-11e3-9d45-12d8877fee74.png)

- [x] on error validation we should get the red border and the tool tip message (create example too)

![image](https://f.cloud.github.com/assets/658431/1231827/7314f0ce-2874-11e3-9737-5cfd1c6c06f1.png)