Shellvar: Add exported and unset as values for ensure
Shellvar: Fix converting unset entries into exported entries
Shellvar: manage un-unset and un-export cases
Shellvar: Remove unset entries on destroy
Shellvar: Use aug.clear instead of aug.touch for ruby-augeas 0.3.0 compatibility
Shellvar: add doc for exported/unset
Shellvar: Fix comments with unset