String io

This is additional work on @arrtchiu https://github.com/fringd/zipline/pull/7 . It adds tests for different file types and also handles Fog files. I'm going to add this to a project I'm working on this week or next so will be able to give feedback if it's actually working for Fog files and StringIO. I'm not using Carrierwave or Paperclip so won't be able to test them other than in the specs.