updated comments for stats-related methods/function to make clear start/end values must be provided in milliseconds
changed use of objects as payload input for json_encode, to associative arrays for consistency
added functions/methods for firewall groups: list_firewallgroups(), create_firewallgroup(), edit_firewallgroup(), delete_firewallgroup()
added checks for required boolean parameters
merged PR by @Warafux:
added edit_client_fixedip() function/method
added optional parameter to list_networkconf() to fetch config details for a specific network
clean up after resolving merge conflicts