Started work on two new image transformations: sharpen and contrast
Added contrast and sharpen transformations
Use the unsharpMaskImage method for sharpening, and introduce gain and threshold params
Added sharpening presets
Fixed indentation
Added docs for contrast and sharpen
Added a unit test for the different presets, and set default values for all params for all presets
Added a test to make sure the preset values can be overridden, and also corrected default values
Updated ChangeLog. Fixes #264 and #265