Change the image animator class to allow non-linear slider axes.

This allows the optional specification of an arbitary array of values
for each slider. This also makes a small and irrelevant change to the
BaseFuncAnimator that makes this possible but does not change the
current behaviour.
A few fixes
Documentation improvements.
Merge pull request #1 from Cadair/imageanimator_tweaks

Documentation improvements.
Merge pull request #1046 from nabobalis/imageanimator_tweaks

Change the image animator class to allow non-linear slider axes