New option to suppress tickLabel tweening

This patch adds a new option named tweenLabels which defaults to true to maintain existing functionality. When set to false the numeric values which are used when a tickLabel is unknown are suppressed.

See example output below:
![screen shot 2013-10-22 at 1 31 29 pm](https://f.cloud.github.com/assets/240521/1385093/feb5c364-3b58-11e3-937f-5befa6d16d42.png)