Mixing colorants with Scales

[Mattriks]

Gadfly currently allows "1 scale per aesthetic", but this causes issues when using some layers with a scaled color aesthetic, and some layers with specified colorants. In the example below plot p1 works, but plot p2 fails. Plot p2 could be fixed by better handling of Colorants in Scale.color_continuous (and more generally in Scale.color_discrete). This would also help other issues (e.g. #1191). If you think this is good, I'll put in a PR.

D = [DataFrame(x=rand(20), y=rand(20), v=g) for g in ["Group1", "Group2"]]
cs = [colorant"blue", colorant"green"]

p1 = plot( Coord.cartesian(fixed=true),
      layer((x,y)->x+y, 0, 1.0, 0, 1.0),
      layer(D[1], x=:x, y=:y, Theme(default_color=cs[1])),
      layer(D[2], x=:x, y=:y, Theme(default_color=cs[2]))
)
p2 = plot( Coord.cartesian(fixed=true),
      layer((x,y)->x+y, 0, 1.0, 0, 1.0),
      layer(D[1], x=:x, y=:y, color=cs[1:1]),
      layer(D[2], x=:x, y=:y, color=cs[2:2])
)

draw(PNG(), p2) # fails
draw(PNG(), p1)