Allow to display Axis.Time labels like in Axis.Category.

Close #1130.

4 different configurations of "center" and "between" layouts

http://jsfiddle.net/kr5eapzt/41/

![screen shot 2014-12-18 at 3 55 44 pm](https://cloud.githubusercontent.com/assets/8551049/5498101/7cfc916e-86ce-11e4-865d-3768b6992c0c.png)

Shoulnd't the 2 arguments be more like
totalHeight and tierHeight ? or similar?

Also, is this._computedHeight === sum(this._tierHeights)?  I don't see why reduce is needed / why we need to store an instance variable for a sum