Added Sortable to families and tasks.

Implements #246.

After running into weird positioning issues with jquery-ui I ended up implementing this using Rubaxa Sortable. Browser support is all modern browsers and IE10+.
There's a few differences that can be generalised on the controller/model side:
- Tasks get set on the task itself, people on the family.
- Task position vs index is off by 1

If we can get all modules to use te same implementation the JS would als be easy to generalise. For tasks i've use acts_as_list to implement te position on the model, or we could make a sortable module.

The remaining question is should setting the position be a concern of the grouping (group/family) or of the item (task/person).