Task hooks

[ismellike]

Closes #76

Adds a new mock-hook-consumer contract to test out all the of the different task hooks.

Adds a test in the task-queue contract to use the mock-hook-consumer.

• The test here is using the open payment config to allow the mock-hook-consumer to create a new task when a task is completed
• When a task is created, the mock-hook-consumer is simply incrementing its created counter.
• When a task is timed out, the mock-hook-consumer will error out, but the error will be handled correctly by the task-queue to prevent blocking.
• When a task is completed, the mock-hook-consumer will create a new task based on the given task's result. (square the square)