Add macro `with_state_machine`

[modocache]

The current API isn't very nice because it requires you to write when: :some_state for each assertion, which becomes tedious quickly.

describe Vehicle
  #3. macros raise exception if specified state_machine is not defined on class
  with_state_machine do # 2. uses default state_machine if no arg is passed
    it { should have_state :parked }
  end

  with_state_machine :alarm_state do # 1. macro sets subject for nested `it` blocks
    it { should have_states :active, :off }
  end
end

• with_state_machine sets the subject to the state_machine defined on a particular attribute. "subject" could either mean the actual subject is shifted to the state_machine instance (or some sort of facade), or perhaps the other matchers such as have_states are aware of the "subject" in some way.
• with_state_machine uses the default state_machine if no argument is passed
• with_state_machine raises an exception if the specified state_machine doesn't exist, or no state_machine exists on the class in the first place