[FEATURE Composable Computed Properties]

This feature allows you to combine (compose) different computed
properties together. So it gives you a really nice "functional
programming" like syntax to deal with complex expressions.

Example:
```javascript
var sort    = Ember.computed.sort,
    union   = Ember.computed.union,
    equal   = Ember.computed.equal,
    mapBy   = Ember.computed.mapBy,
    not     = Ember.computed.not,
    compare = Ember.compare,
    and     = Ember.computed.and;

var Person  = Ember.Object.extend(
               napTime: and(
                          equal('state', 'sleepy'),
                            not('thirsty'),
                            not('hungry')
                          )
              );

var person = Person.create({
               state: 'sleepy',
               thirsty: true,
               hungry: false
             });

var Kitties = Ember.Object.extend(
       allKitties: sort(
               union(
                 mapBy('whiteKitties', 'name'),
                 mapBy('blackKitties', 'name')
               ),
               compare
             )
      );
var kitties = Kitties.create({
        whiteKitties: Ember.A([{
                                 name: 'whiskeys'
                               }, {
                                 name: 'bagels'
                              }]),
        blackKitties: Ember.A([{ name: 'little boots' }]),
       });

person.get('napTime') => false
person.set('thirsty', false)
person.get('napTime') => true

kitties.get('allKitties') => ['bagels', 'little boots', 'whiskeys']
```

Kudos to brilliant @hjdivad for his help.