Add a new initializer and a corresponding setter to set a "default" color

The default color is the color that the slider image views adopts when it initially appears on screen. Before this commit, it was set as clearColor.

The picture is showing an ugly default color of orange.

![photo](https://f.cloud.github.com/assets/1680349/257349/4a6424b2-8c55-11e2-9f07-5709a900c7a0.PNG)