Add callbacks for these three functions - GSM oper status, GPRS status, HTTP request

Line 170 in 72ca8af

sim800_conn_status_t sim800_oper_get_gprs_status ();

Line 141 in 72ca8af

sim800_oper_status_t sim800_oper_get_status ();

Line 164 in 72ca8af

void sim800_oper_http_req (sim800_http_req_t * http_req);

Only if the POST request is success or failure as of now.