Return credits and throttling data with lookup methods

[lpellegr]

As part of a new major version, the IpregistryClient API should be reworked to return a response that includes information about remaining credits and throttling when enabled, in addition to lookup data.

We should mimic what we did with the official TypeScript library:

https://github.com/ipregistry/ipregistry-javascript/blob/0f7120b647e33cf822e042955951f8beaf65709d/src/request.ts#L24

[lpellegr]

Until this issue is fixed, you can retrieve the number of credits remaining on your account as follows:

import json
import urllib.request
resource = urllib.request.urlopen('https://api.ipregistry.co/?key=YOUR_API_KEY')
print('Ipregistry credits remaining=', resource.headers['ipregistry-credits-remaining'])

Note the value for the number of credits is reflected in near real-time.