removed hyphen in iis fields, added mutate block

hyphen in fields isnt allowed and elasticsearch was showing stuff like 'time' rather than timetaken.
mutate block was added in hopes it would enable statistics in elasticsearch (per this thread http://comments.gmane.org/gmane.comp.sysutils.logstash.user/1759 )
Merge pull request #2 from natefox/master

Fixed IIS fields and enabled stats in Kibana for numeric fields