Change intended to improve dygraphs rendering of y-values with tiny values (e.g.

Change intended to improve dygraphs rendering of y-values with tiny values (e.g.
1.0e-7).

* Wrote significantFigures() function for estimating # of significant figures.

* Updated numericTicks() to use significantFigures() to determine how to format
  the y-axis labels.  Altered the return value to include both the ticks and the
  number of sig figs.  Propogated changes to all callers of numericTicks().

* Store result of significantFigures in current dygraphs object and re-use when
  formatting y-values on the legend.  We actually use # sig figs + 1 here
  because we want the y-values to have more precision than the labels; the +1
  gives us gradations in tenths between each tick which should be reasonable.

* Wrote tests/significant-figures.html for checking corner cases of sig figs
  code and rendering with default options.