Boxplot dodging

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• adds dodged boxplots, if aesthetic color is different to aesthetic x
• closes Better support the color aesthetic on boxplots #482
• fixed png color issues in Geom.boxplot outliers (see Cairo backend and svgclass() Compose.jl#339)

Example

using Compose, Gadfly, RDatasets
mpg = dataset("ggplot2","mpg")
p1 = plot(mpg, x=:Class, y=:Hwy, color=:Class, Geom.boxplot, Theme(boxplot_spacing=0.3cx) )
p2 = plot(mpg, x=:Class, y=:Hwy, color=:Drv, Geom.boxplot,
    Guide.colorkey(labels=["front", "4-wheel\t\t","rear"]),
    Theme(boxplot_spacing=0.1cx) )
vstack(p1, p2)

[codecov-io]

Codecov Report

Merging #1246 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1246      +/-   ##
==========================================
+ Coverage    85.6%   85.72%   +0.11%     
==========================================
  Files          35       35              
  Lines        4112     4118       +6     
==========================================
+ Hits         3520     3530      +10     
+ Misses        592      588       -4

Impacted Files	Coverage Δ
src/geom/boxplot.jl	100% <ø> (+7.14%)	⬆️
src/statistics.jl	90.6% <100%> (+0.15%)	⬆️

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[Mattriks]

This is ready to merge.