[Cleanup]
[Manager] Move headers in yarp/manager
[Manager] Move library in yarp/manager C++ namespace
[Manager] Install CMake files

The manager library can now be found (if installed) using

  find_package(YARPManager)

Once found the YARP::YARP_manager target and the
YARP_MANAGER_INCLUDE_DIRS and YARP_MANAGER_LIBRARIES will be available.
Merge pull request #130 from drdanz/ymanager_public

Make YARP_manager library public