add examples/metadata.c

relocates the static functions from metadata.h and adds MetadataCopy()
to the interface

Change-Id: I28bfa9233d3dd70dddf6b561fe0bf4be378db1ec
diff --git a/makefile.unix b/makefile.unix
index 7fcc603..b027786 100644
--- a/makefile.unix
+++ b/makefile.unix
@@ -126,6 +126,7 @@
 
 EX_FORMAT_DEC_OBJS = \
     examples/jpegdec.o \
+    examples/metadata.o \
     examples/pngdec.o \
     examples/tiffdec.o \