Expose yScale in discreteBarChart model

Right now only linear log is set through `discreteBar` inheritance.

What this pull request does is simply add `yScale` to the `discretebar` object, so any `d3.scale` can be used.

![graph using logarithmic scale](https://f.cloud.github.com/assets/37416/2447994/566b24d6-aea0-11e3-8e1a-6cb0b54c5963.png "Logarithmic scale")