Add option to disable zero filling. Closes #612. Closes #934

This adds an option to disable zero filling completely when not in derivative mode

![screen shot 2014-04-10 at 2 45 08 pm](https://cloud.githubusercontent.com/assets/1250387/2673961/c354c10e-c0f9-11e3-8e11-31a7be79abc1.png)

With zero fill:
![screen shot 2014-04-10 at 2 47 01 pm](https://cloud.githubusercontent.com/assets/1250387/2673954/b022c2c0-c0f9-11e3-84af-3450b1a637f6.png)

Without zero fill:
![screen shot 2014-04-10 at 2 45 53 pm](https://cloud.githubusercontent.com/assets/1250387/2673956/b5f0d318-c0f9-11e3-9b8f-f23ac1de4026.png)