Refactor `Site#cleanup`

Move the cleanup logic out into a separate class,
`Jekyll::Cleaner`.
change site_cleaner to use attr_accessor
make obsolete_files a method (@mattr-)
put the Cleaner instance in a `Site#site_cleaner` method
remove no longer necessary condition
move Jekyll::Cleaner to Jekyll::Site::Cleaner
Merge pull request #1429 from maul-esel/refactor-cleanup

Refactor `Site#cleanup` into Jekyll::Site::Cleaner class