Added methods to use Sistrix API

Fixes issue #88.

If my other PR is merged, you can cherrypick the relevant commits from this PR. Thanks!
Yes only on work for on function. In this case your methods was:
- checkApiKey return a bool
- checkApiCredits return bool
- guardApiKey calls checkApiKey and throw a Exception if not
- guardApiCredits calls checkApiCredits and throw a Exception if not

a method with the prefix "check", "is", "have" return generally a bool and don't throw a Exception