@obfuscurity => Add ability to scale charts

I have a few metrics that are increasing in increments that are a very small proportion of the metric, ex: `[3.0, 3.0001, 3.0002, 3.0003...]`. Since the [default minimum of Rickshaw](https://github.com/shutterstock/rickshaw#min) is 0, the graph for this kind of data looks like:

![screen shot 2014-12-05 at 5 04 32 pm](https://cloud.githubusercontent.com/assets/4393191/5323983/932b644a-7ca3-11e4-91fe-0776ff404b71.png)

This PR adds a change where the minimum value on the graph will be the minimum value in the metric y values, if the optional metric arg `scale = true` is set in the dashboard. The same graph will then look like:

![screen shot 2014-12-05 at 5 04 13 pm](https://cloud.githubusercontent.com/assets/4393191/5324005/bf4c1380-7ca3-11e4-8ca1-92ac5cc93341.png)

which is more informative.

Since this is a display issue, I chose to change the min value on the graph rather than manipulate the actual datum with some sort of function like `transform`.
Would you mind changing to this (if it sounds correct to you)? It just reads better to me.

* scale - Use a dynamic y-axis scale rather than defaulting to zero. (optional)