feat(animate): animationsAllowed property to turn off all animations

This is required for writing end-to-end tests against AngularDart apps
that use animations.  In such tests, you typically want to test the
functionality of the application and not necessarily deal with the
animations.  This allows you to turn off all animations in your
production site under test.

The Animate class has a new property, animationsAllowed, which can be
set to false to disable all animations.  When true (the default),
whether or not animations occur depend on the animation module installed
and the AnimationOptimizer.