Patch trim with feathering

Sometimes it is useful to leave a little 'border' around an object untouched when trimming, for example to improve/not mess up a composition too much. The tolerance parameter is not always appropriate. A simple solution is to use resizeCancas() with (for example) a white background after trimming. However this solution is too simple since not all backgrounds are solid.

A  more elegant solution might be to use the concept of 'feathering', also used with the 'magic wand' tool in Photoshop / GIMP, etc. 

Also see item #10: http://www.ehow.com/how_5130229_use-magic-wand-tool.html

This PR adds a (perhaps naive) approach to feathering, some initial tests work nice for me ;). 

TODO: 
- add unit test
- instead of a fixed amount of pixels, using a percentage might be useful, either:
 - of the original 
 - of the trimmed image