Move time-specific options into the time plugin.
Fix incorrect default for xaxes/yaxes tickColor.

The way in which xaxes/yaxes inherit options from xaxis/yaxis resulted
in a minor bug, where tickColor defaulted to the xaxis/yaxis color
instead of the color for its axis.  Fixed by applying the default before
extending the per-axis options, resolving #984.

There's still some questionable behavior here; this section should be
revisited for 0.9, especially with an eye towards removing some of the
code that only exists for backwards-compatibility.