Literal - to disambiguate between string literals and property keys in computed properties

```javascript
import l from 'ember-cpm/macros/literal';
import among from 'ember-cpm/macros/among';

var and = Ember.computed.and;

var MyOtherType = Ember.Object.extend({
  isServerReady: false,
  userInput: 'ready',
  ready: and('isUserReady', 'isServerReady'),
  isUserReady: among(    // NOTE: among doesn't have composed macro support, but it soon will!
    'userInput'          // property key
    l('ready'),          // literal (this would otherwise be ambiguious without using "literal")
    l('good'),           // literal
    l('fine'),           // literal
    l('satisfactory'),   // literal
    l('rad')             // literal
  )
});
```

@jasonmit @stefanpenner @cibernox @hjdivad 
What is the point on supporting `null` and `undefined`?

@hjdivad already did an implementation of this here, and seems simplier: https://github.com/emberjs/ember.js/pull/4185/files#diff-845b71768b2891302a6a3adf45cd55f0R725

Apparently that PR was not merged because of some backward-incompatible changes. I'll wait for his feedback on this.