[BUGFIX beta] Allow Ember.computed.sort to use property paths in sortDefinitions.

'Ember.computed.sort' 'works only if sortDefinitions contain simple properties.
It will fail if a `sortDefinition` contains a property path (i.e. something
like `person.name`).

A `SearchProxy` is used to shadow the new value of an underlying property with
the old value to ensure proper comparisons take place while searching for an
item in the sorted array. When using a property path rather than a simple
property to sort on, `SearchProxy` will use the actual property path in its
attempt to shadow underlying properties - something like: `{ person.name: 'John
Doe' }`. This causes things to blow up because every comparison between an
existing item `get(itemA, sortProperty)` and a SearchProxy `get(itemB,
sortProperty)` will return a bogus result. In this case, `get(itemB,
'person.name')` will not return the shadowed value (stored as `{ person.name:
'John Doe' } `) but rather the current value of that path on the proxy's
underlying `content`.

This commit allows a proper comparison to take place when a sortDefinition
contains a property path.