Fix padding, color, negative axis, and resizing issues in Rickshaw.

1. **Padding:** Top and bottom padding is added to the graph so that tooltips are not cut off.
2. **Color:** @keir raised the valid concern that the `spectrum14` [color palette](http://code.shutterstock.com/rickshaw/examples/colors.html) makes the first series blue and the second series a slightly lighter blue and the third series an even lighter blue. `spectrum14` looks beautiful for a stacked area plot, but is not user-friendly for timeseries. I have selected `munin`, which looks really ugly in the example as a stacked area, but works quite well as a timeseries. It rotates through mostly basic/primary colors in the start. It also features significantly more colors than `spectrum14`, so there will be fewer repeats when many series are present.
3. **Negative axis:** Setting `min` to `auto` allows for negative y-axis values.
4. **Resizing:** The timeseries now takes the full width of the panel, minus the exact space taken by the legend. With upcoming responsive design changes in mind, angular-rickshaw has been modified to redraw the graph when the window is resized.

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Add comment about dependency on underscore.