added additional admin functionality

I've added some admin functionality that I find essential with 'ordered' models, specifically the ability to change object position in list view with up/down links. I've done this by contributing functions to the parent model within the contribute_to_class function. I also added up and down arrow icons as static files.

To use it effectively:

 1) the model should probably (obviously?) be ordered by the `PositionField`
 2) the admin class for the model should subclass or mixin `PositionAdmin`
 3) within the admin `list_display` should include `position_up_down_links`

I can't speak for the efficiency of what I've added but at the very least it will show a good proof of concept of what makes this more useable as an app. It's probably worth having some drag and drop functionality as well and certainly some template tags and readthedocs.org/sphinx documentation to finish it off.