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.
I think I've been confused by this function for quite the while.  Semantically I don't correlate ticks() -> the values on the ticks, I personally correlate them to the actual ticks (the d3 selection).  This somewhat goes along with the setter where I expect that using it will be consistent with the rest of the API where I set something on each tick (again I expected a d3 selection at first glance).

With the above in mind, I am leaning towards splitting the setter and getter in mind since they are not as closely correlated as they appear to be, something to the degree of tickValues() and tickCount(count) (tickCount can also be a getter if we so choose)