sigv4 implemented
No need for expat anymore.
hmac ctx no longer used.
We don't use base64 encoding anymore now that we do signature v4.
Add missing C++ bracket.
Add missing header.
Advertise C++ support, remove expat dependancy, fix typo.
Update TODO.
Batch write item checkpoint, not working yet.
More batch write item progress, checkpoint.
Add another TODO.
More progress on batch write item.
Ignore unprocessed keys working.
Add ignorable.
Add ignorable.
Collecting unprocessed items now, still missing comman handling.
Start writing with new JSON parser.
Batch write item response now working and lightly tested.
Merge branch 'kimdon'
Split reusable json parse functions into a separate file.  Formatting cleanup and add TODOs.