API rate limit management

more and more APIs provide request limit information with either headers or in specific fields in the response

one commonly used place are headers:

```
"X-Rate-Limit-Limit"      => 300,
"X-Rate-Limit-Remaining"  => 298,
"X-Rate-Limit-Reset"      => 2,
```

keep track of those and send requests accordingly to avoid running into limits.
