* made available CustomField class
* added class methods add_customField for various classes
* added class ServiceLevel
* added class History
* added method save_dnsRecord for class IPAM_ipaddress
* added class IPAM_DNSRecord
* change Device42API class method get_assets to get_asset
* change all classes save method to use self._api_path
* added Device42API methods
    - get_service_level
    - get_history
    - get_device
* updated documentation accordingly