Add forget_client (using forget_sta api)

[yuphing-ong]

Nothing new, just cleaning up the lint errors from pull request #43 that stops it from being accepted.

[finish06]

@yuphing-ong Can the block_client and unblock_client also use the new _macs_cmd function?

[yuphing-ong]

@yuphing-ong Can the block_client and unblock_client also use the new _macs_cmd function?

Unfortunately no. FYI _macs_cmd is just sending param 'macs':MACS, where MACS must be a list of macs (one or more). I'm not implementing any iteration, it's just what the Unifi API is expecting.

I'm using https://github.com/Art-of-WiFi/UniFi-API-client/blob/master/src/Client.php as a reference, so block_sta and unblock_sta only take cmd param 'mac' and a single mac, whereas forget_sta takes the param 'macs': '[...]'.