Adds a libsensors based sensors_monitor node

Right now there is only explicit support for Fans and Temperature sensors as the systems I have access to only support those.
However, all other sensors will show up with all of their information there is just no sensor specific summary, units or warning/error support.

Related to #33
This catkin_package() declaration specifies that any package which depends on diagnostic_common_diagnostics should link against a library called `sensor_types`, but I don't see a library here with that name.

We're not build or exporting a C++ library to downstream packages, so the catkin_package() declaration should probably be left empty.