Do not Remove Open Class until Completely Closed

We were removing the ddm-menu--open class too soon so when the menu
closed, there was a display of white-space before the transition
finished.

Delay the removal of the ddm-menu--open class until the menu is
completely closed by listening to transition-end events.
Merge pull request #2 from deseretdigital-ui/closingDisplayNone

Do not Remove Open Class until Completely Closed