Remove unused image processing parameters

This change removes the autoconvert and format parameters. In all call
situations they were either not set or set to the default value anyway.
Pass settings dictionary to generate_image

Reduce the amount of parameters to pass necessary to generate an image.
Add adjust_options

This gives the user the ability to process the images after resizing them. In
particular, the color, contrast, brightness and sharpness can be adjusted.
Update CONTRIBUTING
Merge pull request #30 from matze/adjust-images-after-resizing

Adjust images after resizing