Plot3D does not used current subplot

Not sure if this is a known issue, but the following code gives me this not so nice output. (I'm currently checked out on master.)

``` julia
using PyPlot
figure()
subplot(2,2,1)
plot(randn(10))
subplot(2,2,2)
plot3D(randn(10),randn(10),randn(10))
```


Start by running the issue's Julia example with subplot, plot, and plot3D to reproduce the output. Trace the subplot and plot3D entry points and their interaction with matplotlib. Done means plot3D renders in the currently selected subplot rather than creating or using the wrong axes.

Written by the indexing model from the issue text.