Removed ImageMagick and replaced it with GraphicsMagick

I loved the format and performance of papercut, but was experiencing the following issues:

- When processing dozens of images the thread pool would run out and the program would crash
- About one in ten images would fail due to some bug in ImageMagick
- Occasionally things would slow to a crawl for not apparent reason

When looking into the issues I found numerous people experiencing similar thing in bug reports gone unanswered. The current state of the node ImageMagick plugin is unsupported and unmaintained and the ImageMagick library as a whole seems to have fallen behind. 

As such I proceeded to port papercut over to GraphicsMagick using the plugin 'gm'. Feel free to close this request if it doesn't align with your vision, I just figured I would share seeing as it performs with significantly less errors and headaches. 

Thanks.