Changed cell_deleted to on_cell_deleted, allowed returning false to...

...prevent deletion.

```ruby
def on_cell_deleted(cell)
  if cell[:arguments][:some_value] == "something"
    App.alert "Sorry, can't delete that row."
    false
  else
    RemoteObject.delete_remotely
    # return anything *but* false to allow deletion
  end
end
```