fix memory leak in OpenCL implementation

The lowpass, subbands and scales properties of
backend_opencl.TransformDomainSignal were indeed memoized but memoized
globally meaning that referencing them would keep a reference to the
TransformDomainSignal object indefinitely leading to memory growing
without bound. Replace the @memoize decorator with a simple
attribute-based cache.
update ChangeLog for 0.7.1
release 0.7.1
add changelog for 0.7.2
release 0.7.2
tests: move registration frame loading to utility function

Add a new module, tests.datasets, which will have functions for loading
test datasets.
docs/conf.py: add tests directory to include path
Merge pull request #55 from scf32/overlayquiver

Overlayquiver
docs: add ipython directive extension

Add an extension providing direct execution of Python code samples along
with the ability to embed plots.
docs: add requirements files for building

In addition to the setup.py requirements, we also need to create a
requirements.txt file for the benefit of readthedocs.
docs: add empty _static dir

The automatic plot-embedding support from the ipython directive will
fail unless there is a _static dir available. We don't currently use the
_static dir. Create it and add a .gitignore which ignores gen-*.png
files. As long as we remember to make generated figures match that
filename glob we won't have git status-spam.
docs: add ipython sample to registration
Merge pull request #56 from rjw57/docs-ipython

support ipython execution in documentation
tests: add lena loading function
overlay_quiver_DTCWT: add usage example
move plotting-related functions to dedicated module
Merge pull request #57 from rjw57/plot-module

move plotting utility functions to dedicated module
tests: use datasets.lena() to load lena image

There is tests.datasets.lena(). Use it.
plotting: fix length of quiver key arrows

A speculative fix for #58 which fixes the plot on my (@rjw57) machine. I
don't know if it is the *correct* fix though since I don't know what
units the 'length' parameter has.

Closes #58
plotting: flesh out docs for overlay_quiver_DTCWT