New series recipes for binned data:
* barbins
* scatterbins
* stepbins
New series recipes for histogram:
* barhist (histogram is now an alias for this)
* scatterhist
* stephist
Supports plotting 1D and 2D StatsBase histograms, seriestype can be set to
bar(bins), scatter(bins) or step(bins).
Also adds support for some common auto-binning modes:
* :sturges, :auto - Sturges' formula
* :sqrt - Square-root choice
* :rice - Rice Rule
* :scott - Scott's normal reference rule
* :fd - Freedman–Diaconis rule
Maybe these could be contributed to StatsBase at some point.
Error bars currently don't work correctly for scatterbins and scatterhist,
due to problem with manipulating error bars in a series recipe, but do work
for "plot(h::StatsBase.Histogram, seriestype = :scatter)" (works around
the problem by calling scatter directly, it seems that error bars can be
manipulated correctly in a type recipe).
