CHEF-3714 - Method returns if a file was edited

Implement a method in Chef::Util::FileEdit that returns if a file was edited
This commit contains the method originally written by rhafer@suse.com and
a unit test.
CHEF-3714: Update file_edited? for FileEdit refactor
Merge pull request #1321 from opscode/btm/CHEF-3714

CHEF-3714: add a file_edited? method