Fixed item repitition for multiple items and added omitIfEmpty setting.

The new setting allows building 'cleaner' XML by eliminating empty
elements from exports
Merge branch 'master' of github.com:collectiveaccess/providence
Merge branch 'master' of github.com:collectiveaccess/providence
Improved handling of replacement sheet in mapping XLSX
Added File_MARC PEAR module to our code base for convenience.

Still requires PEAR but gets us around the module install requirement.
Also modified THANKS.txt.