add utility functions for image registration
added image registration example script
registration: added non-highpass sampling functions
examples/registration: add non-wavelet warping comparison
examples: remove wavelet-domain warped image from registration example
utils: re-write reflect to be non-iterative

dtcwt.utils.reflect uses an iterative method to clip the input within
the bounds specified. This in effect makes the runtime proportional to
the magnitude of the input which is an unexpected performance
characteristic.

Taking the implementation from OpenCL, rewrite reflect() to use a
non-iterative, if far more opaque, method.
add implementation of non-rigid registration

This is a first implementation of the local-affine non-rigid
registration algorithm using the DTCWT.
add example of image registration
add tennis imagery to registration example
registration: tune selection of levels slightly

Use one iteration for computing global transform and then never use
above level index 2 (== level 3) for computing registration.
registration: compatibility fix for Py3

xrange is not in Py3. Make use of six's implementation.
update README with changes
docs: add image registration example
registration: document all external functions
Merge pull request #44 from rjw57/registration

Initial implementation of DTCWT-based registration