Add support for ScaleQuestions

This adds support for scale questions, including:
* Randomize the direction of the scale, with support for an anchor
* Multiple rows (attributes) with the same scale
* Randomization goes the same direction for multiple attributes
* Shuffle the ordering of multiple rows

![screen shot 2014-10-15 at 7 00 14 am](https://cloud.githubusercontent.com/assets/4962198/4646554/511879f0-5474-11e4-9a4c-67cdc93e39eb.png)

Issue #17