Balanced.Extension work in progress

I've been working on the Balanced.Extension namespace, which will manage modifications to the application during runtime. 

The API will look like:
```js
Balanced.Extension.register('some-addon', function() {
  this.setTemplate('new-page', /* Handlebars template function */);
  this.extendView('new-page', {
    foo: 'bar'
  });
});
```

Extensions will be registered upon logging in, but not loaded. It is up to the dashboard to determine when to load and unload extensions:

```js
Balanced.Extension.load('new-page');
Balanced.Extension.unload('new-page');
```

Some problems:
- Haven't worked out the process for unloading extensions that aren't the last one yet.
- some helper methods to extend other things may be missing, or I just need to refactor the extension helpers. 

Preliminary extension:
http://github.com/daliwali/balanced-admin