Very basic support for transitions.

I'm certainly not happy with the quality of this, it feels, hacky. I tried to get it to attach more to an issue, but ran into trouble with has_many/belongs_to.
However… it does work for my needs and figure that someone else might also find it helpful.

``` ruby
transitions = client.Transition.forIssue issue_key
transitions.each { |transition|
    # If this is the transition we want to run…
    transition.transitionIssue issue_key
}
```