Move to instancemethods

[jnylen]

This should be better.
@janko-m could you check and comment?

Closes #2

[janko]

Awesome, I think this is a big improvement!

I think it's better in the README to show just regular metadata extraction, so that it's not specific to whether people are generating versions or how they name them (although you can include an example on how to extract only for a specific version as well):

add_metadata :dominant_color do |io, context|
  dominant_color(io.path)
end

photo.image.metadata["dominant_color"]
# or
photo.image.dominant_color

Other than that, everything looks great!

[jnylen]

Thanks @janko-m