Asynchronous HTTP(S) request API.
void get (String url, Function callback, String encoding):
Send an HTTP GET request using url, the callback is called on response. Encoding
is optional (Default: UTF-8).
void post (String url, String postData, Function callback, String encoding):
Send an HTTP POST request using url and the postData, the callback is called on
response. Encoding is optional (Default: UTF-8).