Fix test for astropy 0.4.

Apparently astropy 0.4 now supports the "Z" suffix in ISO timestrings.
Various amendments and fixes.
Implement sign change detection.
Relax precision a bit.

Also document decreased accuracy for sign change detection.
Add new planets and planet methods.

Add Mercury, Venus, Mars, Jupiter, Saturn.
Also revamp angle finder to fit new requirements.
Add some WIP sketches.
Add basic tests for the new functionality.
Update changelog.
Rework angle finder.

Using the local minima of the target distance it's much faster and more
accurate now.

Also rename mod360_fabs to mod360_distance (which is what it actually
does).
Event table generator WIP, plus some minor changes.
Sketch angle finder plotting.