Added support for radio buttons group validation

Validates radio buttons group, and puts the (.xt-error) class and the tooltip on the element that is declared on the (data-val-cont="containerId") of the radio element.

In addition, the code throws an error if the developer forgets to add the (data-val-cont) attribute.

![radio_button_group_validation](https://cloud.githubusercontent.com/assets/2285974/3843323/4fe46956-1e3f-11e4-8c62-c18de7897893.png)