Support Measures units more in x,y scales

[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 more support for Measures units in x,y scales
• fixes a mistake in function cat_aes_var!

Example

• Add text at relative positions on a plot

using DataFrames, Gadfly
import Gadfly: w, h

df = DataFrame(x=[0.6w, 0.6w], y=[0.6h, 0.7h], label=["Text 1", "Text 2"])
layer1 = layer(x=1.0:5, y=[0.21, 0.81, 0.68, 0.76, 0.18], Geom.point)
layer2 = layer(df, x=:x, y=:y, label=:label, Geom.label(position=:right))
layer3 = layer(xintercept=[2cm], Geom.vline(color="orange"))

p1 = plot(layer1, layer2, layer3, Guide.title("Scale.x_continuous"))
p2 = plot(layer1, layer2, layer3, Scale.x_discrete, Guide.title("Scale.x_discrete"))
hstack(p1, p2)