fix($animate): ensure guarded animations consider AJAX requests upon bootstrap

Prior to this fix when an Angular application is bootstrapped it would only
place an animation guard to prevent animations from running when the application
starts for the first two digest cycles. However, if any controllers or directives,
that are executed upon boostrap, trigger any remote code to be downloaded (via $http)
then the guard does not put that into consideration. This fix now properly addresses
that circumstance and removes the guard once all outbound HTTP requests are complete
when an Angular application is bootstrapped.

Closes #8275
Closes #5262
what does "naturally" mean here? can you be more specific?