Escape pathnames for dot

[gak]

In my first test pycallpraph barfed spectacularily (on windows XP/german), because it just fed the pathname for the picture - containing spaces - to dot. The easiest way to fix that is using the subprocess module (in the standard library since python 2.4).

(My version is actually 0.5.1 installed via easy_install but I can't select it under "Version" for this ticket)