more SVG

Hello again,

Just to clean up my mess, I've sent John a couple of patches, one which
cleans up the SVG backend a little, and the other which makes it so that
print_figure() (at least in the agg backends) will save as SVG when the
filename ends in .svg.

A couple other features I had been thinking about, but won't have time
to work on:

-Compressed (gzipped) SVG
-Using <marker> elements for symbols. This would shrink file sizes a
lot. You could create a Polygon collection renderer for this I think,
but it may be more involved...

Also, using python's built-in DOM implementation might result in cleaner
code, but may not be worth it.

Now, I'd better go do some physics before my adviser finds out what I've
been doing with my time... :wink: