feat(rxForm): added new rx-form-fieldset directive

Addresses issue #242 

Some other small tweaks:
1. Removed 'required' attribute from rxFormOptionTable checkboxes
   and made it optional for radio buttons
2. Changed 'th' for rxFormOptionTable input to 'td'
   and wrapped it in a label (so it's clickier)

The following screenshots illustrate changing between rxFormItem to rxFormFieldset. Notice the lack of difference :)
Before:
![image](https://cloud.githubusercontent.com/assets/706039/2964442/86af708a-dae6-11e3-9fe3-682b7c60e500.png)

After:
![image](https://cloud.githubusercontent.com/assets/706039/2964435/7106be64-dae6-11e3-9dfc-2666bd48fb4a.png)