Accept both row and column vectors in plot()

It would be nice if the `plot()` command could accept row vectors as well as column vectors. As it is now, this code doesn't work:

```
julia> x = [1 2 3 4];
julia> y = [2 3 5 8];
julia> plot(x,y)
```

The reason is that Matplotlib is finicky about accepting row vectors vs column vectors. This is a needless limitation in Matplotlib that prevents some simple examples from working correctly. To get around this limitation, PyPlot could/should transpose the vectors as needed before giving them to Matplotlib. It is an easy fix that would make the PyPlot more useful.