Reboot of aiaprep()

New function, affine_transform(), created in sunpy.image.rotate. Currently uses the Crotate extension to rotate an array, but this will hopefully be replaced with the equivalent function in scikit-image.

aiaprep() created in sunpy.instr.aia. It only provides the most basic rotate and recentre functionality at the moment, but it will be improved.
Additional functionality

Added ability to rescale images and to specify recenter and rotation_center keywords, as implemented in previous rotate() and aiaprep() functions. I've tried to rearrange the logic a bit to simplify, but I'm not sure it make more sense. Hopefully it's still doing the same thing though.
Updated a few docstrings and comments
Added option to use skimage AffineTransform for rotations
Option to choose interpolation method partially implemented
Test script for aiaprep() added (currently empty)
First test added to test_aiaprep_rotation()
More test functions written. These will all need some significant tweaking, I expect
Few more tests in the test functions
More tests. A few minor changes to image centre definition in aiaprep().
Adjustment made for center of rotation not being center of image when using skimage warp()
Order of interpolation for warp() increased to 4 (bi-quartic) - it seems to give significantly better results.
Improvements to aiaprep()
Changes to tests
Minor changes to tests
Tests for sunpy.image.rotate.affine_transform() moved to sunpy.image.tests.test_rotate
Tests for sunpy.image.rotate.affine_transform() removed from sunpy.instr.tests.test_aiaprep
test_aiaprep.py renamed test_aia.py for consistency with other test scripts
Assorted clownery. Can't remember what I've changed, but it's closer to being right.
Assorted clownery. Can't remember what I've changed, but it's closer to being right.