Implement "JPEG == small image" rule

- Removes hard-coded "iip" large image rule and puts in a content type
  check instead
- Modifies image factory trait - now you specify jpeg or tiff
- Adds new fixture image just to be sure we can truly test TIFFs
- Fixes downloads spec to use new jpeg factory trait
Merge pull request #152 from OregonDigital/feature/jpeg-is-small-image

Implement "JPEG == small image" rule