Enhancement: TbButtonGroupColumn

Added a wrapper to render TbButtonColumn buttons in a button group. Added a type to the button attributes for ease of choosing the button type ('info','warning' etc)
![Screenshot from 2013-01-02 11:29:27](https://f.cloud.github.com/assets/1700080/37899/b59efe6c-54b7-11e2-871e-7078c49c2fce.png)