Documentation of canBusSkin and embObjSkin devices

[traversaro]

For people working on the skin, it would be really convenient to have a documentation of the parameters supported by the canBusSkin (class CanBusSkin) and embObjSkin (class EmbObjSkin) similar to what we have for the canbusanalogsensor and for the canbusinertialmtb devices.

[traversaro]

cc @joankangro

[valegagge]

Currently you can find how to configure embObjSkin in the robots configuration template files. See https://github.com/robotology/robots-configuration/tree/master/iCubTemplateV3_0/hardware/skin

In the future, we will integrate in doxigen documentation of icub devices.

[randaz81]

We need to document also #477

[pattacini]

The skin-related device drivers will undergo a general overhaul, which will entail that we provide documentation as well.