Update next / done api

* done can return a string - but thats a queue command eg. “stop”
* done can return Meteor.Error or Error - Where Error would throw an
actual error
* null will cause the task to rerun later

[docs](https://github.com/CollectionFS/Meteor-power-queue/blob/master/in
ternal.api.md#doneanywhere)
update meteor 0.8.0