Add ability to grap_draw for drawing onto matplotlib figure

This will allow drawing to mpl figures, this works but is not optimal, the layout is strange, if you run this:

```py
from graph_tool.all import *
from pylab import *

switch_backend('cairo')

fig = figure()
g = Graph()
g.add_vertex(4)
graph_draw(g, mplfig=fig)
fig.savefig('foo.pdf')
```

You will see nodes in the upper left corner.