Added new relevant tags to handle Gazebo plugins

[akhilsathuluri]

The changes were made to load the iCub models from: https://github.com/robotology/icub-models to Drake simulator, which needs some manipulation of the .urdf.

Changes:

• Added new tags relevant to Gazebo (which are eventually ignored)
• Restored adding value and type to the Mechanicalreduction tag within Transmission

[akhilsathuluri]

Now also fixes the issue here: #16

[traversaro]

@akhilsathuluri probably you should avoid commiting the __pycache__ folder.