Provide a more expressive API

[lovell]

Beg, borrow and steal a small but perfectly formed subset of the methods provided by the gm wrapper around GraphicsMagick.

This will make it a bit easier for people to switch to using the multicore-aware libvips library should it provide the features they need.

[mcgoooo]

👍 im really interested in this library, but for one killer feature. the basic needs are the rescrop function found in here.

https://github.com/hacksparrow/node-easyimage

i could try to help, but i can't write c, yet! :D