Use POST method in to retrieve records in iterate

[thomasopsomer]

This is a small patch to address issue like #203, to avoid getting "Request-URI Too Large" error from Airtable API.

I had to add a fct to_json_dict similar to to_params_dict that format options into something accepted by the post method /listRecords

Let me know if you want to use this patch and if you need some change :)

[gtalarico]

Thanks for starting this @thomasopsomer , this was very helpful. I am moving this over to https://github.com/gtalarico/pyairtable/pull/222/