Gazebo YARP plugin for iCub skin

[maxymczech]

Good day. This is an attempt at a pull request to solve issue #55. I apologize for dragging this out for so long. I will describe the idea behind the plugin, and will kindly ask for a review, I have a feeling this will not be accepted at a first attempt.

• This is a model plugin. Upon initialization it subscribes to ~/physics/contacts Gazebo topic and filters out contacts on skin parts, using regular expressions to check for a certain naming convention in collision bodies to determine if this was a contact with skin. I went this road instead of sensor plugin recommended in Gazebo tutorials for performance reasons. As the skin will have hundreds of collision elements, I found model plugin to perform orders of magnitude better than sensor plugin.
• The plugin forwards filtered contact information to corresponding YARP ports at 10 Hz frequency.

As you can see in the header file, I have hard coded body parts and port names configuration. This worked fine for my thesis project, but for the pull to be accepted I guess this has to be configurable via YARP ini files? But I am not sure what is the proper way to do this, can you please point me in a direction for documentation on this?

Other than that, I have studied the code for other YARP Gazebo plugins, but I am not sure if my coding style and conventions are up to your standard.

Once again, I apologize for dragging this issue out.