rework yum.in

Added and enhanced
- data about securty updates
- configurable hiding options
- documentation header for munindoc
- declare own category "updates" for these sort of plugins (should be added to apt etc. also)
- change family to 'manual' as user has to setup a cronjob outside of Munin
Update yum.in

remove superfluous output for option 'update' of plugin yum
yum.in: Fix file encoding

munindoc failed in showing character "å" and warned "POD ERRORS: Non-ASCII character seen before =encoding in 'Manns<E5>ker'."
Seems to be fixed with explicit declaration "=encoding utf8"
Merge pull request #245 from dipohl/devel

rework yum.in