[build] add "docs" target; remove vendor/ diretory on "clean" target
[add] [test] HandleCollection::removeHandle() returns boolean true if handle was in collection and removed, false if handle was not in collection
[fix] HandleCollection::removeHandle() was returning true even if handle was not in collection
[add] [test] ManagerInterface::removeListener() returns true if a handle was removed, false if not
Merge branch 'develop'