Add generated png images for logos (and a Makefile to generate them)

artwork/Makefile

+DEFAULT_DPI=90
+
+all: logo.png logo-wide.png
+
+clean:
+	rm -rf logo.png logo-wide.png
+
+logo.png: logo.svg
+	inkscape --export-png=$@ \
+		--export-dpi=10 \
+		--export-background-opacity=0 \
+		--without-gui $<
+
+logo-wide.png: logo-wide.svg
+	inkscape --export-png=$@ \
+		--export-dpi=45 \
+		--export-background-opacity=0 \
+		--without-gui $<