problem using JavaScript in SharePoint provider hosted app

I have created a provider hosted app for a SharePoint Online site and want to use JavaScript for most of the front end logic, rather than ASP.NET MVC

I have run into Cross domain problems trying to query REST endpoints from the SharePoint Online domain. this article gives advice on getring a basic example up and running for a JavaScript SharePoint provider hosted app, however it doesn't seem to work for me, as I don't have an SPAppWebUrl paramater in the url...

So I tried just using my app domain as the SPAppWebUrl which is something like http://ift.tt/12iMpc6 but it doesn't seem to work, giving the error

GET 404 http://ift.tt/1vyZOIM (Not Found)

That error makes sense to me, as the test app isn't a SharePoint Online site... If I change it to the SPHostWebUrl, it has the correct endpoint http://ift.tt/12iMpsm... but won't work because of the cross domain restriction, giving the error:

Uncaught SecurityError: Blocked a frame with origin xxxx accessing a frame with origin xxx. Protocols, domains, and ports must match.

Here is my code for a very simple JavaScript app to get some list data. The Endpoint opens fine in the browser when I set it to the SPHostWebUrl but not in the app due to the cross domain error

$(document).ready(function () {
    $.getScript(
        "Scripts/SP.RequestExecutor.js",
        continueExecution
      );
});

function continueExecution() {
    // have tried both hostweburl and appweburl 
    executor = new SP.RequestExecutor(hostweburl);
    getCustomers();
}    

function getCustomers() {

    // begin work to call across network
    var requestUri = hostweburl + "/_api/Web/Lists/getByTitle('Customers')/items/" 
    + "?$select=Id,FirstName,Title";

    // execute AJAX request 
    executor.executeAsync({
        url: requestUri,
        method: "GET",
        headers: { "ACCEPT": "application/json;odata=verbose" },
        contentType: "application/json",
        success: function (data) {
            console.log(data);
        },
        error: function (data) {
            console.log(data);
        }
    });
}