Change image processor

[czycha]

Ditch oily_png for the more versatile rmagick, thus unshackling the dependence on PNG I/O files.

[czycha]

It's been a while since I checked up on this project and the Ruby ecosystem of image manipulation. If there's a better library out there post here.

Minimum capabilities:

1. Pixel-by-pixel reading in a standard color format (RGB, RGBA, HSL, HSLA, etc.)
2. Pixel-by-pixel setting in a standard color format

Options that justify ditching oily_png:

1. Wicked fast pixel-by-pixel operations or general performance increase
2. Multiple image formats (PNG, JPEG, GIF)

Unrealistic goals:

1. No external dependencies like ImageMagick

If there are siloed gems for the various image formats (sorta like how chunky_png is for PNG) and they can be switched between in a sane and realistic way, that would be OK but might be a lot of work.

If a different dependency is not viable, should at least upgrade chunky/oily_png.