getXHR.js

function getXHR(url, callback) {
    var xhr = new XMLHttpRequest();       // Creates a new XHR request

    if (!url) {
        throw new Error('No URL supplied');
    }

    xhr.open("GET", url, true);         // Open the connection

    xhr.setRequestHeader("Content-type",
        "text/plain; charset=utf-8;");

    xhr.withCredentials = "true";        // This sends cookies through 

    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4) {      // If the response has finished downloading
            if(xhr.status === 200){      // And the content was found
                try {
                    if (callback) {
                        callback(null, JSON.parse(xhr.responseText));
                    }
                } catch(e){
                    throw new Error('Malformed response');
                }
            }
        }
    }

    xhr.send();                          // Makes the request
}

manifest.json

"permissions": [
  "http://atlas.metabroadcast.com/",
  "http://*.google.co.uk/"
]

postXHR.js

function postXHR(url, data, callback) {  // Added data to function
    var xhr = new XMLHttpRequest();

    if (!url) {
        throw new Error('No URL supplied');
    }

    xhr.open("POST", url, true);         // Changed "GET" to "POST"

    xhr.setRequestHeader("Content-type",
        "text/plain; charset=utf-8;");

    xhr.withCredentials = "true";

    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4) {
            if (xhr.status === 200) {
                try {
                    if (callback) {
                        callback(null, JSON.parse(xhr.responseText));
                    }
                } catch(e) {
                    throw new Error('Malformed response');
                }
            }
        }
    }

    if (data) {
        data = JSON.stringify(data);         // Stringify the data Object literal
        xhr.send(data);                      // Added the JSON stringified object.
    } else {
        xhr.send();
    }
}