fix typos
Add a modifier section in the builder doc
Add a 2D parameter to PoseNoise

If the _2D parameter is set to True
the noise is only applied to x, y and yaw.

Otherwise, the noise is applied to all
data (x, y, z, yaw, pitch, roll).
Add make_ghost method to the Builder

This method makes the robot transparent
and changes its physics type to NO_COLLISION

It allows to show a 'ghost' in the simulator
for instance to display some pose estimation
computed outside MORSE.

A ghost robot has no influence on other
robots (no collision and invisible to laser
scanners), except for video camera that
see the ghost!