Task Groups, Derived State, and .observes()

Changelog

0.6.0

• feature: Task Groups: http://ember-concurrency.com/#/docs/task-groups.
  Task Groups let you enforce concurrency constraints across multiple
  tasks, which previously wasn't possible.
• feature: Moar Derived State: http://ember-concurrency.com/#/docs/derived-state
  Task Instances now expose .value and .error properties for the
  value returned from the task function, or the error thrown from it.
  Furthermore: Task objects expose last and lastSuccessful, which
  point to recently performed TaskInstances, which then make it
  possible to idiomatically access .value and .error, e.g.
  {{myTask.last.value}} or {{myTask.last.error}}. This is
  a continuation of ember-concurrency's goal of exposing as
  much Derived State as possible, and minimizing boilerplate.
• feature: .observes() Task Modifier: appending .observes('foo', 'bar')
  to a task will automatically perform the task when 'foo' or 'bar'
  changes. Thanks to @OFbriggs for co-authoring this feature.
• bugfix: once an object is destroyed, any attempts to perform
  a task on that object will be immediately canceled.