Add "maxTicks" parameter to Rickshaw's time axis.

Accept a new parameter, maxTicks, that allows users to say how many time-axis tick marks should be on a graph.
![maximum tick size is 4](https://f.cloud.github.com/assets/2722306/844850/0df66c7c-f3db-11e2-9e6a-2ba8ddc34487.png)
![no maximum ticks](https://f.cloud.github.com/assets/2722306/844851/0df59310-f3db-11e2-9026-ab44c17e3cfa.png)