a better way to handle the make clean task

Previously, make clean would fail if README.txt did not exist. This is
the recommended way to get around this and it just continues on silently
if there's nothing to be done

Author: ikirudennis

Makefile

@@ -1,6 +1,6 @@
 clean:
-	rm README.txt
-	rm -rf ./dist ./build
+	$(RM) README.txt
+	$(RM) -r ./dist ./build
 
 generate_pypi_README:
 	${VIRTUAL_ENV}/bin/pytextile README.textile | sed -e 's/^\t//' > README.txt