hector_quadrotor: separated gazebo stuff from urdf descriptions

Gazebo descriptions use the .gazebo.xacro extension now.
Building hector_quadrotor_gazebo will fail now if one of the enabled plugins is not found.
hector_quadrotor: switched to new node name robot_state_publisher to suppress deprecated warning in groovy
hector_quadrotor_controller: use sub-elements instead of attributes in gazebo plugin SDF due to a bug in gazebo's SDF parser (see https://bitbucket.org/osrf/gazebo/issue/735/attributes-of-child-elements-are-not)