Add a histogram transformation.

This transformation converts the image into a graph representation
of the color usage in the image. The base size of the histogram is
a width of 256px.

The transformation supports four parameters:

    scale: Allows you to set the width of each bar in the histogram.
           Setting a large scale will allow for anti-aliasing later.
      red: The color to use when drawing the red part of the graph.
    green: Same for green.
     blue: .. and same for blue.

Add a resize/maxSize transformation to resize the resulting image
down to the wanted size if you use the scale parameter and don't want a
size multiplied with 256.
Fix syntax issues with the original pull request.
Make ratio between width and height configurable
Added a mapping for some mime types
Updated ChangeLog
Merge pull request #286 from christeredvartsen/issue-286

Save image/x-jpeg, image/x-png and image/x-gif as image/jpeg, image/png and image/gif
Added a histogram transformation
Removed whitespace
Prepare for a new release
Merge branch 'travis-hhvm-php56' of github.com:rexxars/imbo into travis-hhvm-php56
Steps to get HHVM working