Add optional project_id on get_users end point

[pmcilwaine]

As stated in the https://www.gurock.com/testrail/docs/api/reference/users#getusers Documentation that from TestRail 6.4 and onwards you can pass a project_id parameter to this end point to get only users within the project.

[coveralls]

Coverage increased (+0.09%) to 96.522% when pulling ef58dbfaa4b3ba32000f66457dad21d8e74542a6 on pmcilwaine:feature/handle-project-id-on-users into edb30419d583c5c2d1f36686b33011c038a80af2 on rundef:master.