Unifi-python-api

- fixed exec_curl() for cases where the cookie has expired and we need to re-login
- removed obsolete functions/methods

- updated README with all getter and setter methods/functions
- updated get_cookies() and get_cookie() for consistency
- get_cookie() remains for backward compatibility only, should not be used in new code
- fix rare cases where the client would end up in a loop, e.g. issuing a request for a site the cached credentials have no access to

- removed several unnecessary if statements
- added getter and setter for unifi_os property
- fixed logout method when working with UniFi OS-based controllers

- improvements to internal use of $_SESSION['unificookie']
- other minor code improvements
- applies several applied patches as suggested by scrutinizer-ci.com

- applied patches to Client.php as suggested by scrutinizer-ci.com
- moved resource checks to get_curl_resource() method/function
- extended create_user() function/method with several optional parameters
- changed headers that are passed with each request containing a payload

- applied patches to update_switch_poe-mode.php as suggested by scrutinizer-ci.com
- code clean up and improvements in preparation of support for UniFi OS-based controllers
- added support for UniFi OS-based controllers, thanks to Scyto for providing access
- adapt login route and method, and base URL for UniFi OS-based controllers
- automatically identify UniFi OS-based controllers, thanks to TwitchCaptain for this and several other suggestions
- relaxed URL validation to allow UniFi OS-based controllers to pass
- changed default HTTP method to GET
- many improvements throughout the code