Axis and tick color options now make more sense.

The axis color now controls the color of the axis line, instead of its
ticks and labels, while the tickColor controls the tick color. This
makes a little more sense and provides the minor feature of being able
to change the axis line color separately from that of its ticks.  Pull
request #917 ought to be easier to merge now, too.
Update version number to 0.8.0-beta.