contrib: add new temperature script

This script uses the raw output to support any sensor chip.

[vd: squash and move to contrib]

Closes #30
scripts: (cpu_usage) use default error message

die() already provide a very similar default message.
readme: notice the ruby-ronn dependency

This tool is needed to manually generate the man page.

Closes #56
config: document the default config file

Add the list of valid properties in the default config file, as well as
a link to the manual page.