more automatic rate limiting

[maelle]

Fix #9

This PR

• renames the .quick_fetch function as it's not really quick. Its name no longer starts with a dot, as preparation for Consider exporting .fetch_results #82
• uses ratelimitr for limiting the rate.

The default rate limit (30 requests per slot of 10 seconds) is what I've observed in headers. I update it after every API call. All of this because the Meetup API docs do not document how the rate could change. https://www.meetup.com/meetup_api/#limits

Example of code that would fail with the current package version but is automatically paced with this PR

for (i in 1:50) {
  lala <- get_events("rladies-nashville")
  print(Sys.time())
}

[maelle]

One thing that worries me with our default app is whether other people could get requests throttled.

E.g. for the website it might be a better idea to create an OAuth app specific to the website. I wonder how many OAuth apps we can create with our pro account.