Add Transition API handling

Added TransitionService with 2 methods:
 * GetList for retrieving possible transitions for an issue
 * Create for creating transition and changing issue status in the
   process

jira.go

@@ -28,6 +28,7 @@ type Client struct {
 	Issue          *IssueService
 	Project        *ProjectService
 	Board          *BoardService
+	Transition     *TransitionService
 }
 
 // NewClient returns a new JIRA API client.
@@ -55,6 +56,7 @@ func NewClient(httpClient *http.Client, baseURL string) (*Client, error) {
 	c.Issue = &IssueService{client: c}
 	c.Project = &ProjectService{client: c}
 	c.Board = &BoardService{client: c}
+	c.Transition = &TransitionService{client: c}
 
 	return c, nil
 }