Completely refactored the code structure and implemented the FOLLOW animation

The source code was cleaned up and more code was moved into separate classes. The CGazeboProxyObject class now holds information identifying objects in a Gazebo environment (model name) as well as holding state variables (pose, twist). The CGazeboSpawnableObject class holds the absolute path (modelFile) of an object's `.model' file for spawning purposes. This is done automatically for the action camera when starting the node.

Also, the service interface was slightly changed in favour of functionality and simplicity. As such, an offset for objects to watch can now be specified but is not mandatory.
Corrected a few field types
Exported a few parameters to the launch file
Fixed a broken parameter type in the launch file
Added informative output onto the console when calling node services