feat($breadcrumb): add the customization of the parent state with a function

The `ncyBreadcrumb.parent` property can be a string containing a state name
of a function returning a state name :
```
$stateProvider.state('B', {
  url:'/b',
  ncyBreadcrumb: {
    label: 'State B',
    parent: 'B'
  }
})
```
or
```
$stateProvider.state('B', {
  url:'/b',
  ncyBreadcrumb: {
    label: 'State B',
    parent: function() {
      return 'B';
    }
  }
})
```

The function can have a $scope argument : it will be called with the same
scope used by interpolation in labels:
```
$stateProvider.state('B', {
  url:'/b',
  ncyBreadcrumb: {
    label: 'State B',
    parent: function($scope) {
      return $scope.parentStateName;
    }
  }
})
```

Closes #32