onewire plugin: Add a plugin to read onewire sensors.

onewire plugin: Add a plugin to read onewire sensors.

This plugin allows to read _temperature_ sensors connected over a
onewire bus using the `owcapi' library from the `owfs' project. Other
sensors can be added easily, but without hardware to test this it's
kind of hard to be sure it all works.

ToDo's: Document the plugin, make the `Alias' configuration option work.