Create some example extensions for the item overview

[Nicklas Nordborg]

BASE 3.2 will add extension points for adding node loaders and validators to the item overview (​http://base.thep.lu.se/ticket/1680 and ​http://base.thep.lu.se/ticket/1681). We should include some examples in the example code.

1. Include the user that is the owner of an item as a property node.
2. Continue to load the role membership of that user as child nodes.
3. A validator that checks if the user has a valid URL address or not.
4. A validator that checks if the user is a member of at least one role.
5. An information extension that display some additional properties for the selected item. We could, for example, display more or less the same as the toolbar button example.

[Nicklas Nordborg]

(In [1602]) References #377: Create some example extensions for the item overview

Added loader implementations for User and Role.

[Nicklas Nordborg]

(In [1603]) References #377: Create some example extensions for the item overview

Added validation implementations for user and role that check the URL for the user and role membership.

[Nicklas Nordborg]

(In [1604]) Fixes #377: Create some example extensions for the item overview

Updated README with the latest information.