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+.
The JS in this file and in tasks.js.coffee is close enough, I wonder if we could:

1. make a generic `sortable.js.coffee`
2. add a generic `sortable` class and `data-reorder-path` to the parent table

Is there anything that is specific to the lists being sorted?