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!
Problem is, that if I use a guard method and I'm checking there if the API key is present and if I have enough API credits (by using the API), I run into endless recursion. So I think I need two guard methods, one for the API Key, one for the API credits. Or what would you suggest?