IO_HTTP: Support creating group data with `send_group_data`

[tyeth]

Fixes #97

@brentru as mentioned in discord, I had a quick look at this while in the codebase, and this seemed reasonable. I also fixed the path for send_batch_data. That then lead me to poke my nose in and test batch method (requiring from collection import namedtuples), I'll leave that for now

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[tyeth]

Just a note that it occurred to me to test the created_at field from device (failed due to RTC not set) so I moved to using io.receive_time(), which I then thought to test from another country with crazier time zone offset (NZ) which failed.

I've therefore added an optional timezone argument to the receive_time method and updated the demo to use it. Tested tz argument to integration with GB and UTC, blank string didn't work (therefore we only add the tz argument if necessary)