Add story transitions

[tir38]

This will add story transitions end point. Finishes #86

I'm not certain how to get VCR to generate the right .json.

[coveralls]

Coverage decreased (-0.3%) to 96.181% when pulling 6fa5232 on tir38:add_story_transitions into d0b5962 on dashofcode:master.

[forest]

This should be named transitions so that it is an override of the getter. The getter is implementing a lazy load of the transitions.

[tir38]

Cool. So the test .json is now generated. But the story that's used doesn't have any transitions:

request:

"request":{  
            "method":"get",
        "uri":"https://www.pivotaltracker.com/services/v5/projects/1027488/stories/66728004/transitions",
            ...

response:

"response":{  
            "status":{  
               "code":200,
               "message":null
            },
            ...

Do I need to add a transition to that story, or should I rely on a another story which has transitions?

[coveralls]

Coverage decreased (-0.07%) to 96.371% when pulling bea84d6 on tir38:add_story_transitions into d0b5962 on dashofcode:master.

[forest]

I'll figure out the tests. Thanks for contributing.

[forest]

@tir38 pulled you code into #90 and got the test passing. Just have to use a story that had some transitions. To do that I had to log into the Tracker project and transition the story. VCR just records the API calls so they have to work for real at least once.