The size of CloseButton in MetroTabItem will be changed proportionately.

The size of CloseButton presented in MetroTabItem will be changed proportionately considering its HeaderFontSize. In addition, an example has been added into MetroDemo project.

![screen_shot_2014-04-19_at_5_33_36_pm](https://cloud.githubusercontent.com/assets/1088621/2748088/2bbef874-c7a6-11e3-87bf-adf53165f136.png)