Pull Request for Issue #272

Fixed issue #272. Code taken from  [Mechanize::File#save](https://github.com/sparklemotion/mechanize/blob/master/lib/mechanize/file.rb#L54-60) was used to create allow creation of parent directory in the function [ Mechanize::Download#save](https://github.com/sparklemotion/mechanize/blob/master/lib/mechanize/download.rb#L58-59).