Stat.ellipse and Geom.polygon updates

[Mattriks]

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

This PR:

• adds updates for Stat.ellipse and Geom.polygon
• enables linestyle aesthetic for Geom.polygon
• Stat.ellipse auto-maps its levels argument to linestyle
• updates documentation

Example:

using Gadfly, RDatasets
set_default_plot_size(21cm, 8cm)
D = dataset("datasets","faithful")
D[:g] = D[:Eruptions].>3.0

coord = Coord.cartesian(ymin=40, ymax=100)
pa = plot(D, coord,
    x=:Eruptions, y=:Waiting, group=:g,
    Geom.point, Geom.ellipse,
    Theme(lowlight_color=c->"gray") )
pb = plot(D, coord, Guide.ylabel(nothing),
    x=:Eruptions, y=:Waiting, color=:g,
    Geom.point, Geom.ellipse(levels=[0.95, 0.99]),
    Theme(key_position=:none, lowlight_color=identity, line_style=[:solid,:dot]) )
pc = plot(D, coord, Guide.ylabel(nothing),
    x=:Eruptions, y=:Waiting, color=:g,
    Geom.point, Geom.ellipse(fill=true),
    layer(Geom.ellipse(levels=[0.99]), style(line_style=[:dot])),
    Theme(key_position=:none) )
hstack(pa,pb,pc)

[codecov-io]

Codecov Report

Merging #1234 into master will increase coverage by 0.26%.
The diff coverage is 94%.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage    85.1%   85.37%   +0.26%     
==========================================
  Files          35       35              
  Lines        4103     4096       -7     
==========================================
+ Hits         3492     3497       +5     
+ Misses        611      599      -12

Impacted Files	Coverage Δ
src/statistics.jl	89.41% <100%> (-0.04%)	⬇️
src/geom/polygon.jl	91.42% <89.28%> (+24.76%)	⬆️

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