d3.geo.circle: ensure coincident start/end points.

Due to floating point rounding error, d3.geo.circle would sometimes miss
out the end point, which should be coincident with the start point.

Fixes #1476.
Fix #1483; time parsing with padding modifier.
Rephrase test.
Simplify.
Remove major class from axis ticks.

The major class is now obsolete with the removal of tick subdivision.
Merge remote-tracking branch 'origin/time-parse-padding' into 3.3.1
Merge remote-tracking branch 'origin/geo-circle-precision' into 3.3.1
Bump version.