Tasks order in job result array

[claustres]

In previous versions tasks in job result array were ordered similarly as input tasks but following #151 tasks in job result array are now ordered according to their run duration: if a task finishes before another one it will be put before it in the result array.

This can causes problem to some hooks like mergeJson, which rely on the task result order. We should probably introduce a new sortBy hook when a specific order needs to be enforced or reorder the result array the same way input tasks are.

[claustres]

Fixed by adding a sortBy option in the mergeJson hook as it's probably better to make each hook requiring a specific order to setup it.