Merge branch 'hotfix/0.3.1'

* hotfix/0.3.1:
  Add json gem for 1.8.7
Merge branch 'release/0.4.0'

* release/0.4.0:
  Fix regressions.
  Store image in subdirectories.
  re-adding spec for large-image detection
  re-implement overwrite spec.
  Refactoring: move image-stuff to new Thumbnail class
Keep generated images.
Merge branch 'hotfix/0.4.1' into develop

* hotfix/0.4.1:
  Keep generated images.