feat(modal): add option to disable animations

Note: Move backdropClass logic into compile function because otherwise
modifying classes in the compile function is broken when using an interpolated
class attribute.

Fixes angular-ui#1007
Closes angular-ui#2725

Author: chrisirhc

src/modal/docs/readme.md

@@ -36,3 +36,6 @@ Finally, a `modal.closing` event is broadcast to the modal scope before the moda
 preventDefault on the event, then the modal will remain open.  The $close and $dismiss methods return true if the 
 event was allowed.  The event itself includes a parameter for the result/reason and a boolean parameter that indicates
 whether the modal is being closed (true) or dismissed.
+
+The `modalConfig` exposes the following global option for all modals/backdrops:
+