Make YARP_manager library public

The manager library is currently a static/private library built inside yarp and linked internally by the yarpmanager, gyarpmanager, and gyarpbuilder.

Since we are working for porting the GUIs to Qt5, but we want to do this in a separate repository (see https://github.com/robotology-playground/qtyarp), we need to export this library in order to link it from outside.

This patch makes the library public when the `YARP_EXPERIMENTAL_MANAGER_LIB` is installed.

The library was moved to a proper namespace (`yarp::manager`) and includes in `include/yarp/manager`.

This library is _not_ included in YARP exported libraries, in order to avoid to have it magically linked to all the executable that link `${YARP_LIBRARIES}`
In order to use it you have to do:
```cmake
  find_package(YARP_manager)
  include_directories(${YARP_MANAGER_INCLUDE_DIRS})
  target_link_libraries(<target> ${YARP_MANAGER_LIBRARIES})
```