add webactivities support to packaged app and handlers for e.me (bug 883402)

https://bugzilla.mozilla.org/show_bug.cgi?id=883402

# Usage:
```javascript
// Look up app by its manifest URL.
new MozActivity({
    name: 'marketplace-app',
    data: {manifest_url: 'http://littlealchemy.com/manifest.webapp'}
});

// Look up app by its slug.
new MozActivity({
    name: 'marketplace-app',
    data: {slug: 'littlealchemy'}
});
```