C++14 -> C++11 ; added missing deps

No C++14 feature was used anywhere, and C++11 is much more widely
supported across current stable compilers.
Also this package depends on `dynamixel_msgs' and
`dynamixel_controllers'; this dependency is now specified in
CMakeLists.txt, fixing the build.
Removed dep on apparently non existent headers

The following headers weren't found in any installation of dynamixel_controllers:

dynamixel_controllers/SetComplianceMargin.h
dynamixel_controllers/SetCompliancePunch.h
dynamixel_controllers/SetComplianceSlope.h
dynamixel_controllers/SetThreshold.h
dynamixel_controllers/SetTorque.h
dynamixel_controllers/SetTorqueLimit.h
dynamixel_controllers/TorqueEnable.h

All code still compiles and runs, even after removing all the respective #includes.
merged branch hdr

The branch `hdr' consisted in the removal of a few superfluous headers
in gazebo_ros_dynamixel_motor.h, and the fixing of CMakeLists.txt (added
a couple of missing dependencies, build didn't work).