Add on_finish argument to Task constructor

[edublancas]

Related to #14

Currently, the only way to add an on_finish hook to as ask is:

task.on_finish = some_callable

Even though that API is fine, it should also be possible to add it in the Task's constructor:

Task(..., on_finish=some_callable)

[edublancas]

We want to keep constructor parameters at minimum, there isn't any strong reason to have all hooks as init parameters when they can be set via properties. closing ths.