Clean field names

Non-clean field names cause problems with drawing, e.g. when there are spaces
or dashes in the process name. This commit cleans the field name by using the
appropriate support module. For more information, see:

http://munin-monitoring.org/wiki/notes_on_datasource_names
Added documentation for 'Clean field names'
Merge remote branch 'mjholtkamp/master' into devel