Simple initialization

More simple initialization. You can initialize by pass the API key string
to fastpay function.

fastpay("YOUR_API_KEY", function (err, res, body) {
    if (!err && res.statusCode === 200) {
        console.log(body);
    }
});

Also, passing the option object works for backward compatibility as follows.

fastpay({
    apiKey: "YOUR_API_KEY"
}, function (err, res, body) {
    if (!err && res.statusCode === 200) {
        console.log(body);
    }
});
Bump version to 1.1.0