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)

[ueco-jb]

Good and clean example of how to implement hooks.
Depending on the level of devs who are supposed to read from that example, you might want to explicitely mention in the readme that the cw-minus hook's interface is being used here (if that's not obvious :) ).