- added information to several docblocks
- changed handling of request headers, added private property for this
- applied changes for improved use of require_once in the README and examples
- added `list_sites()` example
- *potential breaking changes:*
renamed `get_request_type()` and `set_request_type()` functions/methods to `get_request_method()` and `set_request_method()` respectively
- 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
