feat(valdrFormGroup): add new directive for valdr validated form groups
- add new valdrFormGroup directive which sets validity state for a group of form items and is responsible for adding and removing validation messages if valdr-message is loaded
 - change class names used by valdr on form group level (default is now ng-invalid/valid instead of has-error has-success,this can be overridden by changing the properties on the valdrClasses value)
 closes #11, fixes #44, fixes #48
chore(demos): update demos with new valdrFormGroup directive