Support std::function for callback.

[kekyo]

Worked on my ESP8266 environment ;)

class PedestrianSignalButton
{
    // suppressed...

    void requested()
    {
        digitalWrite(PUMPED, HIGH);
        pPlayer->play(WAIT_SOUND);
        delay(500);
    }

public:
    // suppressed...

    void Init(DFRobotDFPlayerMini* pPlayer)
    {
        this->pPlayer = pPlayer;

        // Use std::function for callback the class member.
        requestButton.onPressed([&]() { requested(); });
        requestButton.begin();
    }

[evert-arias]

@kekyo Hi. I'm sorry for the delay. Your pull-request has been merged to the master branch.
Thank you very much for your collaboration.

Best regards