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.
I think this bit here that actually manipulates the data should be moved to the `table_data.rb` file. https://github.com/clearsightstudio/ProMotion/blob/master/lib/ProMotion/table/data/table_data.rb

We already have a `delete_cell` method in there, so maybe we could call the new method `move_cell` or something.