There is a temporary workaround to add your own favicon using the site's metadata.
We've had a lot of requests for this feature lately and will include it in an upcoming update.
In the meanwhile you can use this method to add your own favicon to your website.
You'll need to upload your favicon to a photo hosting service that will give it a public url (like http://postimage.org/), and then copy the link to it.
Go to the Pages tab of the main toolbar and then click the Manage Site Icon.
Paste the following code into the Site Meta Tags field:
<link rel="icon" type="image/ico" href="[paste the link between these quotation marks]"/>