Reorderable table screen cells

Pass `:moveable` in the cell hash as `true` and then put the table into editing mode by either calling `toggle_edit_mode` or `edit_mode(enabled:true)` and you'll see the draggable handles.

When you drop the cell, it will fire `on_cell_moved:` with arguments of a hash:

```ruby
{
  :paths   => {
    :from     => #<NSIndexPath:0xb777380>,
    :to       => #<NSIndexPath:0xb777390>
  },
  :cell    => {
    :title        => "Whatever",
    :moveable     => true
    # Your other cell attributes
  }
}
```

this should be ready to go and has great test coverage.