- added static getTeamLeaderIds method to job object which extracts and returns an array of team leader ids from an array of jobs
- added optional parameter to getTimeslot method in TimeslotService function to verify that timeslot is actually accessible (can use method mentioned above)

TODO: verification does NOT protect against timeslots that are in general accessible but already booked by others!
Merge branch '2.0' of https://github.com/AIESECGermany/myExperiencesOnline into 2.0