Improve tick-count setting API.

Now, ticks(count) sets the number of ticks and returns the
calling QuantitiveScale, instead of setting the number of ticks
and returning that many ticks.

API breaking change.

Close #1057.
Adjust the ticks API to be closer to D3

Added the numTicks(count) call to set the default number of ticks.
A different number from the default can be requested with ticks(number),
which no longer changes the default.

Now less API-breaking, although the functionality is slightly different.
Use default arg assingment in ticks().
Merge pull request #1060 from palantir/1057-tick-count-API

Improve tick-count setting API.