in Rest_Versions :
 - add parameter "sharing" as describe here : http://www.redmine.org/projects/redmine/wiki/Rest_Versions
 - add constants to describe "status" and "sharing" parameters.
     * status: the status of the version in: open (default), locked, closed
     * sharing: the version sharing in: none (default), descendants, hierarchy, tree, system
Integration test for project version's "shared" parameter.