getDependantPropertyKeys bug fixes

This fixes two issues with `getDependantPropertyKeys` and closes https://github.com/cibernox/ember-cpm/issues/103.

#### Keys can now contain spaces

The following no longer raises an exception:

```javascript
Em.Object.extend({
  fruit: 'blood orange',
  isABloodOrange: allEqual('fruit', 'blood orange')
});
```
![image](https://cloud.githubusercontent.com/assets/2526/4964176/96a3eb10-6745-11e4-865b-4b2cfe449161.png)

#### Literals are now supported

Previously, the following error would be raised:

```javascript
Em.Object.extend({
  fruit: 'blood orange',
  isABloodOrange: allEqual('fruit', l('blood orange'))
});
```

![image](https://cloud.githubusercontent.com/assets/2526/4966922/3a50a99a-67ed-11e4-851c-cd091a33d11b.png)