changed create_wlan function/method to not include passphrase in payload when security = "open" and passphrase = null
fixed indentations in several places
added custom_api_request() function/method which allows a programmer to issue custom API requests that may not yet be supported by this client (to be used with care!)
added example to demonstrate use of custom_api_request()
removed repetitive code and optimized exec_curl() when creating the final payload, resulting in 10-20% performance increase and slightly lower memory consumption by PHP
