New method getDestinations(sess)

- added new method getDestinations(PerunSession sess) into ServiceManager
- added into RPC enum
getExtSourceAttributes moved to getExtSourceById

- getExtSourceAttributes erased and all its functionality is now in
  getExtSourceById
- added new mapper for attributes from table ext_sources_attributes
- setAttribute method added into ExtSource object
Added test for getDestinations
Merge pull request #432 from Natrezim/extSourceRebase

getExtSourceAttributes() moved to getExtSourceById()

- getExtSourceAttributes() erased and all its functionality is now in getExtSourceById().
- added new mapper for attributes from table ext_sources_attributes.
- setAttribute() method added into ExtSource object.
Merge pull request #406 from Natrezim/getDestinations

New method getDestinations(sess)