add index V1 format, a direct translation of internal dict

Python encode/decode libs work directly with dicts, so the internal dict
can just be passed directly to any of these libs (pyyaml, pyjson, msgpack,
simplejson, etc).  This still generates the exact same index.xml as before.

This converts the internal format for the repo timestamp to a datetime
instance, which can be easily converted to UNIX time in seconds for XML
and UNIX time in milliseconds for the new index formats.  UNIX time in
milliseconds is directly serialized into a java.util.Date instance by
Jackson.

fdroidserver/common.py

@@ -2192,5 +2192,7 @@ def is_repo_file(filename):
             'index_unsigned.jar',
             'index.xml',
             'index.html',
+            'index-v1.jar',
+            'index-v1.json',
             'categories.txt',
         ]