MATLAB-like drawnow().

About a week or two ago, I was looking for an easy way to update the figure during a long time-intensive iterative process. The first results I found were matplotlib's animation framework and that being way to complex, I instead tried looking for a way use a MATLAB-like drawnow. I couldn't find one, so I wrote one myself.

I tried merging this into matplotlib, but it wasn't mpl kosher. This seems to be the correct place to contribute this function. If not, let me know the correct place (if there is one).

A typical use case:

```python
N = 16
z = zeros((N,N))

ion()
for i in arange(1*N):
    z.flat[i] = 1
    drawnow(draw_fig_real, show_once=False)
```

Known limitations, parameters and a usage case are all covered in the docstring.