[1283] Parent Menu Items

For #1283
With these changes you can pass settings to ```parent``` to customize your parent menu, which is especially useful when you have a catch-all parent menu that doesn't have its own page (so you can't configure it there).

```ruby
menu :label => 'Users', :priority => 0, :parent => {
  :label => 'Administration',      :priority=> 3,
  :url => 'that_place/over_there', :id => '...?' 
}
```
#### UPDATES
* adoped @gregbell's proposed syntax, allowing `:parent` to be a string, proc, or hash of options
* my first commit broke the (already broken) parent label proc functionality from #1664 (only evaluates once)
  * my second commit added it back in, this time in a way that the proc gets re-evaluated on page load

#### TODO
* write proper documentation, including:
  * @rainchen's solution for parent menus as an alternative
  * explanation that `id` is necessary to associate multiple child menus under a parent using a proc for its label.