Log Message:
Changes 1.5:
* Better support for multiple source generators.
* Update method used to check for modification dates of source and thumbnail
  images. Thanks Ben Roberts.
* Better thumbnail_high_resolution handling, including the ability to switch on
  and off explicitly with a ``HIGH_RESOLUTION`` thumbnail option.
* Added configuration option to specify the infix used for high resolution
  image handling.
* Optional postprocessor for image optimization. Thanks Jacob Rief!
* More remote storages optimization
* Thumbnail dimensions can now optionally be cached. Thanks David Novakovic.
* New ``zoom`` option to generate a thumbnail of a source image with a
  percentage clipped off each side.
* New ``background`` source processor that can add a border color to ensure
  scaled images fit within the exact dimensions given.