Aiaprep completed

Proper (hopefully) testing for the aiaprep() function that's been in development for quite some time, and for the associated affine transform.

While mostly finished, this still needs a few things to be done to it (ie: @Cadair still wants a few things done to it):
* [x] Modify Map.rotate() to use image.rotate(), then change aiaprep() to call Map.rotate()
* [x] Remove the C code for the Crotate function and delete relevant bits from setup.py
* [x] Normalisation of images for the skimage warp() function may not be general enough to always work - there's probably a much cleverer way of doing it.
* [x] Improve docstrings
* [x] make sure docs are being built
* [ ] add more tests, and make them pass travis.
we could call it `use_scipy` because the test for the importability of scikit-image is what `force_scipy` is all about. If skimage is not importable then we have to use scipy.