Fixed colors in colors-tab not changing with theme

So cleaning up on the mess from [#989](https://github.com/MahApps/MahApps.Metro/pull/989), the colors now change with the theme again.

![image](https://f.cloud.github.com/assets/764517/2093695/48c871e2-8ebd-11e3-8430-485bd28ddd01.png)
![image](https://f.cloud.github.com/assets/764517/2093699/5a035300-8ebd-11e3-8785-935712573079.png)

The only way that I found to manage this, was to introduce a new behavior, `BindableResourceBehavior` that I have included in MahApps.Metro core library.  I tried to make it a attached property instead, but failed. I have used the base class `Behavior<>`. If referencing the namespace `System.Windows.Interactivity` is undesired, I will have to change that later.