Added modal fade out animation.

ModalWindow.visible now uses ModalInstance._visible
ModalInstance._visible now controls the .in class of .modal instead of the display property of modal-window. 
Thus ng-class is no longer needed.
Removed ng-class

ng-class is not needed anymore because the .in class is now controlled by ModalInstance._visible
Merge pull request #111 from just95/patch-5

Added modal fade out animation