Merge pull request #146 from kt10aan/master

OpenNI2 Driver fixes and improvements
Fix CMake variable name
[cmake] replace remaining add_library calls with yarp_add_plugin

These calls have been deprecated for a long time.
Clean-up motivated by #147
Never call the overloaded version of add_library for IMPORTED libraries

This allows calling find_package inside YARP_{BEGIN,END}_PLUGIN_LIBRARY
blocks, for packages using add_library(IMPORTED)

CC-Issue: #147
Cleanup
yarp_opencv does not use ACE_LIBRARIES
Move find_package(OpenCV) in YarpFindDependencies

CC-Issue: #147
Fix lowercase argument
warnings fixed in SoundFile.cpp
Check ENABLE_yarpmod_opencv_grabber instead of SKIP_opencv_grabber

CC-Issue: #147
a couple of warnings fixed
cast integer type in debug message
one too many arguments in debug message
[ros] fix wire type for compiled ROS types

See #149, thanks @miaragao
Implemented NetworkClock using native YARP constructs
Fixed bug where now() returned shared t instead of local result

Also optimized the implementation following some of @MirkoFerrati suggestions
The delay function cannot oversleep now
avoid use of std::list + std::pair in header file

The std classes cause linking pain, especially in MSVC land.
Cross-reference #150
add Thread::getKeyOfCaller, static version of Thread::getKey
fixed missing kff parameter in pid structure