Use SVG plots in OctaveMagic by default due to lack of Ghostscript on Windows Octave

Newer builds of Octave on Windows do not include Ghostscript, and do not support printing PNG files out of the box.  I suggest making the default SVG files, which is supported.

As of now, a user would have to follow the instructions here to enable support for PNGs:

http://wiki.octave.org/Octave_for_Windows#Printing_.28installing_Ghostscript.29.