Support duck typed commits and switch to Rspec

[lserman]

I didn't like how UpAndAtThem::Transaction ensured that it's tasks were UpAndAtThem::Commit's. I made it so that UpAndAtThem::Transaction will just call #call and #rollback on the tasks without worrying about type.

This makes it possible to do stuff like

class ExamplePayment
  def call
    # logic to make a payment
  end

  def rollback
    # logic to refund a payment
  end
end

UpAndAtThem::Transaction[ExamplePayment.new]

I think it makes sense to put the payment logic in it's own class and without having to worry about wrapping it inside a UpAndAtThem::Commit.

Also, I moved the tests to Rspec since that's what most devs at Metova are familiar with (ok ok... it's because I couldn't figure out how to run the Minitest tests...)

[NotDaveLane]

Thank you!