Bug 1021481 - Handle app:// protocol for DEBUG=1

...ut port change
Here, we should try to experiment getting rid of httpd completely.
Instead of mapping to `uri = 'http://' + appId + ':' + GAIA_PORT + fileSpec`,
we can try mapping to build_stage with something like this :
```
let STAGE_DIR = Services.prefs.getCharPref("extensions.gaia.stage_dir"); // to add from preferences.js...
uri = Services.io.newFileURI(utils.getFile(STAGE_DIR, appId)).spec;
```
And see how it goes.