add ability to set number of xAxis ticks

[djbordelon]

Like gantt.tickFormat(), this change wraps d3.svg.axis.ticks() so that the user can set the ticks they want to see on the x axis.

• a single integer will approximate that number of ticks, e.g. d3.gantt()....ticks(12) will place approximately 12 ticks (the default behavior of d3.svg.axis is .ticks(10)).
• or, a d3.time interval can be used, e.g., d3.gantt()....ticks(d3.time.month, 6) will place a tick for every six months.
• calling gantt().ticks() with no arguments will return the current ticks setting.

.ticks() is an optional call, so, as with d3.svg.axis, this change should not effect existing implementations that do not use .ticks().