Minor new features and bug fixes

• setLimits now used to set any kind of restriction to memory usage. It replaces numberOfConnections and limitStorage. Now, also number of processed messages per second or the amount of processed information can be limited, so that the app will not get stuck after receiving too many messages.

• onClose callback added to the openPage function. It allows specifying actions to be taken when a WebSocket connection is closed (user closes the page).

• getPort added as a function and as a method of class App to query for a port number of the running app.

• sendData now handles the NAs properly.

• Issue with failing to establish a WebSocket connection to a complex address fixed.