Prevent browser cache when adding children to content items

Some browsers (ahem, IE) cache more aggressively and using angular js $resource fails to get new content when making AJAX requests.   

When adding new child content items, IE fails to update the list of children in the N2 admin left nav list.

This fix adds a "cache busting" parameter when fetching children from the content service to ensure that the new items show up immediately and the user isn't all "where's the thing I just created?".