Closes #68

Changes

Add kernelcare_license_key, redis_password, redis_memory_limit, nodejs_version, mariadb_version, mariadb_cluster_name, mariadb_backup_interval, postgresql_version and postgresql_backup_interval attributes to the Cluster model.
Add optimizing_enabled and backups_enabled attribute to the Database model.
Add group_poperties to the Node model.
Add Tombstone endpoint.
Add ApiUsers endpoint, which is just a basic endpoint that returns the raw data.
Add the children call to the Clusters endpoint, which is just a basic endpoint that returns the raw data.

Checks

[x] The version constant is updated in Client.php
[x] The changelog is updated (when applicable)
[x] The supported Cluster API version is updated in the README (when applicable)