Added timeout to ModalWindow.attach

The action of setting ModalWindow.animate to true must be deferred by a few milliseconds such that the .in class is added after .fade.
Update modal.dart
Renamed animate to visible

Renaming animate is less confusing and makes its relation to .in more obvious.
Renamed ModalOptions.animate to preventAnimation

The name animate implied that the option enables animations, but it actually prevents them.
The default value was set to false.
Merge pull request #109 from just95/patch-3

Fixed modal animation