rm path from the hubo_stand plugin filename

- the full path is not portable
- the shared library should be in the ld.so path anyway
re-add the hubo_stand plugin

It has been removed in the 94301da0358e commit. I don't know
what the reason was but I need the controllers.
update the hubo_state plugin: joint name changed

it changed in the 94301da0358e commit
model.urdf: fix whitespace
fix the robot model: don't let it loose its head in Gazebo

The robot seemed to be headless in the Gazebo simulator.
This was probably due to the physics simulation. Neck-related
links had zero mass. This probably leads to "infinite acceleration"
related effects.

Solution: set non-zero inertial parameters for neck-related links.

From Gazebo related tutorial:

    "For the Gazebo physics engine to work properly, the <inertial>
    element must be provided as documented on the URDF link element
    page. For links to not be ignored in Gazebo, their mass must be
    greater than zero. Additionally, links with zero principal
    moment of inertia (ixx, iyy, izz) could lead to infinite
    acceleration under any finite torque application."

see
http://gazebosim.org/wiki/Tutorials/1.9/Using_A_URDF_In_Gazebo#.3Cinertial.3E_Element
make head-related joints that are not driven fixed

do not let the robot's head fall on side