Fix 2011/zucker make clobber, update .gitignore

Remove and ignore *.ppm, *.png.

2011/zucker/Makefile

@@ -207,7 +207,7 @@ clean:
 
 clobber: clean
 	${RM} -f ${TARGET} ${ALT_TARGET}
-	${RM} -rf *.dSYM
+	${RM} -rf *.dSYM *.png *.ppm
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \
 	    echo 'ate sandwich'; \