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)

I'm not liking this variable/map at all.  renderTierLabels is returning a variable meaning that it is rendering as well as returning useful information.  Do we really want to render here?  If so, why does the variable namesuggest that we just have the tierTicks.  Why is a state-affecting function inside a map?