RecordArray#length not dependent on the _ids property anymore

This PR fix issue #28. 

RecordArray#length based on the `_ids` property when available leads to
an array with null values and not Ember.Model instances. The length property is now computed from the array content (i.e. ArrayProxy#length). 

The idea was to provide a length property different than zero when RecordArray was not finished loading. @ebryn, there might be a good reason for your first implementation, please review the changes in my PR. 

See http://jsbin.com/aroxed/2/edit for more details.