Implement Mobile Express Checkout (MEC)

Add an option to enable mobile device support using MEC.

It's a simple option to return the PAYMENTURL with 'cmd' argument to
redirect url to '_express-checkout-mobile'.

Usage:

    var cred = {
        username  : config.paypal.api_username,
        password  : config.paypal.api_password,
        signature : config.paypal.api_signature
    };

    var opts = {
        sandbox : true,
        mobile_express_checkout : true,
        version : '93'
    };

    var ec = new PayPalEC(cred, opts);

    ...

MEC details
https://developer.paypal.com/docs/classic/mobile/gs_MEC/