Implement Mobile Express Checkout (MEC)

[sogko]

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/