Version 1.21

• Add getInstance() methods to ensure only one instance of the driver in an app and make constructor private
• Tidy up demo app
• Update build tools

    Mididriver midi = MidiDriver.getInstance();
    // or
    Mididriver midi = MidiDriver.getInstance(this);  // Add listener
    // or
    Mididriver midi = MidiDriver.getInstance(() ->  // Use lambda expression
        // Do whatever
        );