add zoneinfo metadata
Add the entire zoneinfo metadata to the tar archive, and add
a function to access it. Also add the metadata to the docs.

Fixes #27  instead of #39
diff --git a/updatezinfo.py b/updatezinfo.py
index ed688de..f33e42e 100644
--- a/updatezinfo.py
+++ b/updatezinfo.py
@@ -26,7 +26,8 @@
         sha_512_file = sha_hasher.hexdigest()
         assert metadata['tzdata_file_sha512'] == sha_512_file, "SHA failed for"
     print("Updating timezone information...")
-    rebuild(metadata['tzdata_file'], zonegroups=metadata['zonegroups'])
+    rebuild(metadata['tzdata_file'], zonegroups=metadata['zonegroups'],
+            metadata=metadata)
     print("Done.")
 
 if __name__ == "__main__":