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!
lets throw the exception by the getter

[call example](https://github.com/ClemensSahs/SEOstats/blob/hotfix/semrush/SEOstats/Services/SemRush.php#L107-L108)
[methode example](https://github.com/ClemensSahs/SEOstats/blob/hotfix/semrush/SEOstats/Services/SemRush.php#L229-L236)