Add y-axis log scale to the graph widget

[ke4roh]

With this graph widget, it's now trivial to get a log-scale y-axis.

• It automatically switches from linear to log if values are all on one side of zero and the minimum and maximum are at least 500x apart.
• It can be switched manually by specifying data-yscale="log" or, instead of "log", "linear", or specify the base, e.g. log(2), log(e), or log(10).
• Y-axis labels are even powers of the base.

This also upgrades d3 and rickshaw to the latest.

[terraboops]

Are there any breaking changes in the upgrade of rickshaw? Does this commit integrate with #542?

[ke4roh]

I've been running it for 3 months and haven't noticed any changes (aside from it working) with the upgrade to Rickshaw. It will not play well with #542 because it references an even newer version of d3.