Add example program for Curve25519

Getting a lot of questions about how to use it. This will hopefully get people
started.
diff --git a/programs/Makefile b/programs/Makefile
index 544a2d8..443689b 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -49,6 +49,7 @@
 	hash/hello$(EXEXT)		hash/generic_sum$(EXEXT)	\
 					pkey/dh_client$(EXEXT)		\
 	pkey/dh_genprime$(EXEXT)	pkey/dh_server$(EXEXT)		\
+	pkey/ecdh_curve25519$(EXEXT)					\
 	pkey/ecdsa$(EXEXT)		pkey/gen_key$(EXEXT)		\
 	pkey/key_app$(EXEXT)		pkey/key_app_writer$(EXEXT)	\
 	pkey/mpi_demo$(EXEXT)		pkey/pk_decrypt$(EXEXT)		\
@@ -112,6 +113,10 @@
 	echo "  CC    pkey/dh_server.c"
 	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) pkey/dh_server.c   $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
+pkey/ecdh_curve25519$(EXEXT): pkey/ecdh_curve25519.c $(DEP)
+	echo "  CC    pkey/ecdh_curve25519.c"
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) pkey/ecdh_curve25519.c   $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
+
 pkey/ecdsa$(EXEXT): pkey/ecdsa.c $(DEP)
 	echo "  CC    pkey/ecdsa.c"
 	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) pkey/ecdsa.c       $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@