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.
draw_grid improvements
Time-dependent assumptions when missing metadata (distance to Sun and apparent solar radius)
PEP8 indention fixes
Merge pull request #1046 from nabobalis/imageanimator_tweaks

Change the image animator class to allow non-linear slider axes
Changelog entry for draw_grid changes
Changed print to warnings.warn_explicit
Merge pull request #1048 from ayshih/draw_grid

[Review]: Fixing `draw_grid`
Added same behavior for carrington_longitude and heliographic_latitude
Changed import of sunpy.sun functions
Merge pull request #1047 from ayshih/metadata

Alternate approach to missing location-related metadata