Partially implemented the extents sample

I need to figure out a way to provide the inner *mut
bindings::PixelWand reference to the MagickWand method, and must sort
out how to structure the logic so that nested closures aren't required.
Updated Makefile to better declare dependencies
Changed borrow to use closures rather than procs

This gets around a problem I faced with the Send semantics of procs
and trying to nest them.
Hooray for rust visibility rules!