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.
![no maximum ticks](https://f.cloud.github.com/assets/2722306/844868/4ffecf38-f3db-11e2-863c-674a3d8718d7.png)
![maximum tick size is 4](https://f.cloud.github.com/assets/2722306/844869/5015d714-f3db-11e2-8fe6-34c995c90b97.png)