fixes bug where exception raise if parent is not available

I found that when using the example of a scrollable list from http://blog.nodejitsu.com/a-blessed-ui-for-jitsu, I would get an exception during list.setItems() if the list was not already added to the screen - I wanted to create the lists and add and remove them to the screen in response to user input.

This gist reproduces the problem: https://gist.github.com/johnspackman/6192788

The fix is in the commit.