Assert in the event that a non-POPAnimation instance is provided to a…

…dd(plan:).

Summary:
The POPAnimationPerformer presently only supports plans of the type POPAnimation. If a new plan type is mapped to this performer then the performer will now fail loudly as a reminder to handle this new type.

Closes #8

Reviewers: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Reviewed By: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D1583

src/PopMotionFamily.swift

@@ -48,7 +48,7 @@ extension POPAnimation: Plan {
   }
 
   func add(plan: Plan) {
-    guard let animation = plan as? POPAnimation else { return }
+    let animation = plan as! POPAnimation
     guard let token = self.willStart() else { return }
 
     tokens[animation] = token