make callbacks optional for task.success and task.error functions

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This week of work on @hoodiehq is sponsored by Cargo Media.

   Their slogan is “Small Team, Big Impact” and they are hiring
   a Frontend and Backend Dev: http://go.hood.ie/cargo-media
bump version

* * *

This week of work on @hoodiehq is sponsored by Cargo Media.

   Their slogan is “Small Team, Big Impact” and they are hiring
   a Frontend and Backend Dev: http://go.hood.ie/cargo-media