Add decrypt_item method to Chef::Kinfe::FileDecrypt

This method abstracts the Chef call used to decrypt an item from an
encrypted data bag. The API changed in Chef versions 10.18.0 and
11.0 -- See: http://tickets.opscode.com/browse/CHEF-3392
Merge pull request #2 from racker/chef_10_18_decryptor_fix

Add decrypt_item method to Chef::Kinfe::FileDecrypt