API to add plots and trees

This API is meant to be wrapped by actual view functions. For instance,

```
def edit_plot(request, instance, plot_id):
   plot = get_object_or_404(Plot, pk=plot_id, instance=instance)
   try:
      return render_to_response('finished-editing-plot.html', { 'plot': updated_plot_and_tree(request, plot)})
   except ValidationError as v:
       return Http500(v)
```