Add Firefox OS manifest

This adds a [Firefox OS manifest](https://developer.mozilla.org/en-US/docs/Web/Apps/Manifest?redirectlocale=en-US&redirectslug=Apps%2FManifest).

The result looks like this:

![screenshot from 2013-08-28 13 24 03](https://f.cloud.github.com/assets/1447206/1044748/6fab14e2-1017-11e3-9338-5d5dd4821a49.png)

There's still some work to make this nice on Firefox OS:

  * We could create an icon that fits the [icon guidelines](https://www.mozilla.org/en-US/styleguide/products/firefoxos/icons/) better.
  * The login page can't scale small enough.

    ![screenshot from 2013-08-28 13 31 28](https://f.cloud.github.com/assets/1447206/1044808/6d6e3e6a-1018-11e3-88fe-df5688f865dd.png)

  * The front page can't scale small enough (I made /login the "main page" to work around this).

    ![screenshot from 2013-08-28 13 29 59](https://f.cloud.github.com/assets/1447206/1044801/5673286a-1018-11e3-9f5a-a07392c3dcc2.png)

I think this is a good first step though, since it sets up a reasonably good icon, and once logged in, everything works as expected.