fontsize px bug.

write_svg had a duplicated "pxpx" in the fontsize, so that the font did not appear at all. 

Further improvements could be done here:

- allow fname to be either a string or a file handle, so that one can pass sys.stdout. This is generically a nice thing to do, if you want pipe the output somewhere, say to imagemagick  "display".

- allow for stroke width of the edges.