added transition, works for limited use case

This allows the application of transitions on issues.
It does not have any working tests, so I don't expect it to be accepted. However, if you would like to write some, be my guest. I will be using my fork internally, and I will hopefully get around to testing this code soon.

Example usage:
```
# issue = a jira-ruby issue
transition = issue.transitions.build()
trans_response = transition.save(:transition => {:id => 51})
```