Adds appversion rendering to footer

I almost always add a version number display to my apps for various reasons: people can refer to it when encountering bugs, it's an easy confirmation the application was rebooted after a release, etc.

This PR introduces a new setting `appversion` which is rendered (together with `brand`) in the footer.

Usage examples:

```js
//file: keystone.js
keystone.init( {
  'name'  : 'd-pac',
  'brand' : 'd-pac',
  'appversion' : require("./package.json").version
} );
```

or

```js
keystone.set('appversion', "0.5.4");
```

Which will get rendered as (on a retina resolution):

![footer](https://www.evernote.com/shard/s59/sh/8c857513-1f2b-44ae-a371-29451331471c/4e8de93435c941ba9eacf35cc323dfe7/res/bc5a9ed3-5bac-4f2e-a2ed-1e25e659b576/skitch.png)

Sidenote: TBH I think it would be better to store an `app` object, sth like:

```js
'app' : {
  'name'  : 'd-pac',
  'brand' : 'd-pac',
  'version' : require("./package.json").version   
}
```

But I'm a bit hesitant to make the necessary changes, since it's a bit hard to track down all places where `name` is used for instance ( a search will yield 1000+ results).