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

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)
Merge pull request #250 from rackerlabs/rxFormFieldset

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