- 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
changed variable name $url to $path in custom_api_request() for consistency and clarity
rearranged layout of Client.php
added check to verify whether $ch is a resource
renamed get_curl_obj() to get_curl_resource() to reflect the variable type it returns
Added the new functions/methods:
* set_super_mgmt_settings_base()
* set_super_smtp_settings_base()
* set_super_identity_settings_base()
