Fix color assignment in 2D Histogram (Fix #310)

[darwindarak]

I think color information in the 2D histogram is being stored in row-major order, while the bincounts are stored in column-major order.

xdat = [1, 2, 3, 1, 2, 3, 1, 2, 3, 1]
ydat = [0, 0, 0, 2, 2, 2, 4, 4, 4, 2]
plot(x=xdat,y=ydat, Geom.histogram2d(xbincount=5, ybincount=5))

Gives the following

as opposed to

This might not be the best way to fix this, but transposing bincounts before assigning the color values seems to work. So

plot(x=randn(100000),y=randn(100000), Geom.histogram2d(xbincount=100,ybincoun
t=100))

Now correctly shows

[coveralls]

Coverage remained the same when pulling dc74ad5 on darwindarak:master into 672a6d8 on dcjones:master.

[dcjones]

Great, thanks for fixing this!