Change the default background to black (#25)

* change background color to black

If the page doesn't load fast enough, a white screen will show on the
desktop because the default backgroundColor is white. Changing it to
black makes the screen easier on the eyes before the HTML page is
pre-loaded.

* ignore build folder generated by build.sh

.gitignore

@@ -0,0 +1,2 @@
+/build
+

package/contents/ui/main.qml

@@ -27,6 +27,7 @@ WallpaperItem {
         anchors.fill: parent
         url: wallpaper.configuration.DisplayPage
         zoomFactor: wallpaper.configuration.ZoomFactor
+        backgroundColor: "black"
         onCertificateError: function (error) {
             if (wallpaper.configuration.InsecureHTTPS) {
                 error.acceptCertificate()