docs(interval.js) alerting the developer

Many times $intervals are used inside of directives or controllers. If a directive/controller is destroyed, then the $interval should be destroyed as well. The interval created by this service does not have a way to track an associated $scope. This could cause some issues with developers who assume that the $interval will be cleared for them when the $scope is destroyed.

I believe that the library could/should handle this as well, but thats another issue.