Allow `recordIsLoaded` to be called with a string for the type.

`recordIsLoaded` did not support execution with a string as the `type`
argument, only a DS.Model class reference. This commit makes either
form possible. Consistent with `find`, the `type` argument is
passed through `modelFor` to resolve a class reference.
Merge pull request #1536 from lukemelia/record-is-loaded-with-string

Allow `recordIsLoaded` to be called with a string for the type.
remove unneeded closure.
Ember.computed's first property can also be the DK
Fix property order in code comment

The property order should be `type` -> `hash` -> `prop`.
Merge pull request #1538 from tricknotes/fix-property-order-in-code-comment

Fix property order in code comment
Removing deleted records from RecordArrays is now async.

Previously filtered record arrays were updated asynchronously when records were added, but record arrays were updated synchronously when records were deleted.  This led to surprising behavior such as:

```js

myRecordArray.get('length') // => 2
myRecordArray.invoke('deleteRecord');

// we didn't delete everything, because we mutated the record array during iteration
myRecordArray.get('length') // => 1
```

Now all record array updating goes through the same code path.

-- David J. Hamilton & Stefan Penner
Merge pull request #1541 from stefanpenner/async-record-array-deletion

Removing deleted records from RecordArrays is now async.
Ember.run.once of an anonymous function is just wasteful, as each
invocation creates a new function.

All this would do is add some extra work to backburner.