perf + sync start

0.8.0

• POSSIBLE BREAKING CHANGE: the internal task scheduler
  has been rewritten to be more performant, but to also
  more immediately start executing task functions. Prior
  to this version, perform()ing a task wouldn't actually
  start executing the task function until the actions queue
  on the run loop; this behavior was inconsistent with the timing
  of async functions, and meant that certain lifecycle hooks
  that depended on logic being run synchronously couldn't be
  used with ember-concurrency Tasks (because they already
  missed their window of execution). This is unlikely to
  break anyone's apps, but it's possible some apps out there
  have subtle timing dependencies on tasks running within
  run loop queues, so it's better to announce this as a possible
  breaking change. (#107)
• Derived state: Task Instances now have an additional
  isSuccessful and isError property (#119)
• Derived state: Tasks expose performCount that tracks
  how many times a task has been performed.
• waitForEvent and waitForQueue for pausing the task until
  a jQuery / Ember event occurs, or until a particular
  run loop queue has been reached.