Fast coordinate transformations

[Stéfan van der Walt]

Hi all,

I just added a PR which provides a "warp" function, with signature
`warp(image, transformation_function, ...)` where the function would
be, e.g.:

def transformation_function(xy):
    xy[:, 0] *= 2
    return xy

This is very similar to ndimage.geometric_transform, except that it is
blazingly fast because it is built on top of the much faster
ndimage.map_coordinates.

With this change it becomes trivial to implement transformations such
as swirl (the included example), ripple, the log-polar transform, etc.
by writing the appropriate transformation_functions.

I'd appreciate a review: https://github.com/scikits-image/scikits-image/pull/184

Thanks!
Stéfan