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.

Start at the plot() entry point and trace how Julia vectors are passed to Matplotlib. Reproduce the row-vector example from the issue, compare it with column vectors, and consider the behavior complete when both forms plot correctly without requiring callers to transpose them.

Vom Indexierungsmodell aus dem Issue-Text verfasst.