[grub] add Grub4DOS support

* Also adds some stubs for Grub 2.0
* Also add a note about VS2013 CE in the readme
* Addresses the first part of #340

src/drive.c

@@ -567,6 +567,8 @@ const struct {int (*fn)(FILE *fp); char* str;} known_mbr[] = {
 	{ is_syslinux_mbr, "Syslinux" },
 	{ is_reactos_mbr, "ReactOS" },
 	{ is_kolibri_mbr, "KolibriOS" },
+	{ is_grub_mbr, "Grub4DOS" },
+	{ is_grub2_mbr, "Grub 2.0" },
 	{ is_zero_mbr, "Zeroed" },
 };