gazebo_plugins: moved initialization of shared_ptr members of
GazeboRosCameraUtils to GazeboRosCameraUtils::Load()

This fixes segfaults in gazebo_ros_depth_camera and
gazebo_ros_openni_kinect as the pointers have not been initialized
there.
gazebo_plugins: use shared pointers for variables shared among cameras

It is not allowed to construct a shared_ptr from a pointer to a member
variable.
Merge pull request #120 from meyerj/fix-gazebo-plugins-segfaults

Segfaults in camera gazebo plugins due to uninitialized shared pointers