Reprojection of images.

Both ndarray to ndarray and inter-dataset reprojections are
possible. TODO:

- Add the finer warping options.
- Use valid data masks when warping.
- Test data types other than uint8.
- Make warping of big datasets chunky. Though we do have the
  option of rasterio dataset windows.

Lots of improvements to be made. Much cargo, very culting. I've
still a lot to learn in the intersection of Cython, C++, and
GDAL's warping API.

Closes #12.