fix(modal): allow for custom user modal sizes

Remove restriction to Bootstrap-native `modal-sm` and `modal-lg` size classes.

Fixes angular-ui#3429
Closes angular-ui#3431

Author: RevanProdigalKnight

template/modal/window.html

@@ -1,3 +1,3 @@
 <div modal-render="{{$isRendered}}" tabindex="-1" role="dialog" class="modal fade" ng-class="{in: animate}" ng-style="{'z-index': 1050 + index*10, display: 'block'}" ng-click="close($event)">
-    <div class="modal-dialog" ng-class="{'modal-sm': size == 'sm', 'modal-lg': size == 'lg'}"><div class="modal-content" modal-transclude></div></div>
+    <div class="modal-dialog" ng-class="size ? 'modal-' + size : ''"><div class="modal-content" modal-transclude></div></div>
 </div>