Added option for placehoder TreeItems that do not link to anything.

Added option for placehoder TreeItems that do not link to anything.
This allows for items in the tree that appear in breadcrumbs and menus, but do not actually link to anything.

Added placeholder boolean field to ItemTreeBase model.
Altered admin forms to show field.
Altered templates to not renter link if item is placeholder.

See issue #143 for motivation.

One PROBLEM: 
The URL field still requires a value, since I did not feel comfortable adding a null=True to the url field and adding checks depending on item.placeholder in the forms.
So I have added the note in the help_text of the placeholder field saying: A URL is still required, but will be ignored.

Guess that should be quite clear.