Pause execution if request limit reached

[judith-bourque]

Check in the response if the request limit was reached and pause for the amount of time provided in the response.

This will help respect the API policy:

"The returned HTTP header X-Rl contains the number of requests remaining in the current rate limit window. X-Ttl contains the seconds until the limit is reset.

Your implementation should always check the value of the X-Rl header, and if its is 0 you must not send any more requests for the duration of X-Ttl in seconds." (source)

Closes #122

[judith-bourque]

Before last commit, it still threw a too many requests error

[judith-bourque]

Sometimes it works for 50 requests, other times it throws a too many requests error message. Could there be a limit per hour for free usage?

The documentation only mentions a limit per minute for free non-commercial usage.

[judith-bourque]

The throttle works as intended.

I suspect that the 429 errors were because I was using a shared IP adress with other researchers. Multiple calls could have been made without my knowledge.