WIP: Add promises returns for deferred calls

[RickyCook]

• Return promises from deferUntilTransition
• Resolve promises with return value from this.handle on processQueue

This allows values to be passed out of the FSM before a state transition happens:

fsm = machina.Fsm({
  ...,
  states: {
    uninitialized: {
      '*': function() {
        this.deferUntilTransitation();
        this.transition('start')
      }
    },
    start: {
      getValue: function() {
        return 'the value'
      }
    }
  }
})

assert await fsm.handle('getValue') === 'the value'

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This change is 

[RickyCook]

Unsure how you'd like this handled. I figured that for the moment I'd be safer, and just skip the test if Promise isn't available but this might not be the best option.

[RickyCook]

This breaks existing tests for the defer* functionality because there's an extra property there. Is there any way in the test suite that we can ignore this property? I couldn't find anything in the should.js documentation, but I've never used that lib before so I may have missed something