Autoscale does not work for artists added with Axes.add_artist

Adding an artist through Axes.add_artist() does not update the data limits of the plot, so autoscale does not work.

``` python
ax=gca()
circ = Circle((0,0), 5)

ax.add_patch(circ)   #autoscale works for this
#ax.add_artist(circ)  #autoscale does not work for this

ax.relim()  # not strictly necessary
ax.autoscale(True)
plt.show()

```
