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 
Addressed here:
https://github.com/jamesarosen/ember-cpm/pull/84/files#diff-6e2aa07a15d0d1243940d377e7d1a41eR45