Add API rate limit support

[davidchchang]

This has been a very useful tool for exporting a small number of Quip documents, but I found if you have a significant number of folders and subfolders, I would frequently be throttled by the Quip platform. It would be ideal if rate limiting was baked in (they seem to rate limit at around 50 calls per minute, but this may vary per account) so requests would not fail with a 503.

[sonnenkern]

Please, look at this package: https://github.com/sonnenkern/quip-export
I've succsessfully solved the HTTP 503 "Over Rate Limit" problem. If the application receives the 503-answer from API, the workflow just wait, and repeat the query, wenn it is possible. The data is not being lost.