Add a distclean target for Make

This runs a `git clean -fxd` to revert to a pristine Git clone.

Makefile

@@ -8,6 +8,8 @@ clean:
 	rm -rf .eunit
 	rm -f test/*.beam
 
+distclean: clean
+	git clean -fxd
 
 depends:
 	@if test ! -d ./deps; then \