Unifi-python-api

- added information to several docblocks
- changed handling of request headers, added a 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

- changed references to *UbiOS* back to *UniFi OS*
- removed capitalization from all header strings (per RFC, header fields are case-insensitive: https://tools.ietf.org/html/rfc7230#section-3.2)
- removed charset parameter from headers (not required per RFC)
- added x-csrf-token header to all requests except GET when talking to UniFi OS-based controllers, thanks go to paciks for raising 86

- added several monthly stats methods: `stat_monthly_gateway()`, `stat_monthly_site()`, `stat_monthly_aps()`, `stat_monthly_user()`, thanks go to Roel Janssens for spotting these MongoDB collections
- test for `object` or `resource` in `get_curl_resource()`, closes PR 82 submitted by banakito
- applied changes to comments in `example/modify_smartpower_pdu_outlet.php` based on a suggestion by thesohoguy
- removed unnecessary variable from `list_apgroups()`, thanks to Stephen Davies for reporting
- added an optional parameter $ap_group_ids to the `create_wlan()` function/method for UniFi controller versions 6.0.X and higher, thanks to Stephen Davies for contributing

- minor improvements of comments contents
- fixed naming error in comments for list_devices()
- updated supported controller versions in the main README
- change variable names used from MBytes to megabytes for improved consistency

- minor syntax improvements based on Scrutinizer CI feedback
- added support for PATCH requests (for future use)
- completed updating method/function comments to PHPDoc format (PSR-5)

- added methods/functions to manage AP groups: list_apgroups(), create_apgroup(), edit_apgroup(), and delete_apgroup() (only supported with controller versions 6.0.X and higher)
- adapted code to handle the behavior of the new v2 API routes that were added with controller versions 6.0.X and higher
- merged PR 76 which adds methods/functions check_controller_update() and check_firmware_update(), contributed by brenard
- further work on updating method/function comments to PHPDoc format (PSR-5)