hmac: Implementation of HMAC in glib

This implements g_hmac_xxx() functionality using the standard checksum
functions supported by glib.

HMAC is a secure way to hash a key and a password. Many other
approaches fraught with append and prepend issues.

Includes test cases defined in relevant RFCs

https://bugzilla.gnome.org/show_bug.cgi?id=652480

glib/glib.h

@@ -48,6 +48,7 @@
 #include <glib/gerror.h>
 #include <glib/gfileutils.h>
 #include <glib/ghash.h>
+#include <glib/ghmac.h>
 #include <glib/ghook.h>
 #include <glib/ghostutils.h>
 #include <glib/giochannel.h>