Update Local.pm

Was 'escape_filename' written as a function instead of a method on purpose, or was it an oversight?

This causes problems when trying to sub-class this to create another local flat-file storage area.
Merge pull request #100 from jesusbagpuss/patch-2

(More informative commit messages would be appreciated)

Use a class method for escape_filename() in Storage::Local allowing sub-classing of that specific method.