arrayProxyFor resolves manyArray.
Add DS.PromiseArray#load

promiseArray.then() resolves when we have the ids (links not supported yet)

promiseArray.load() resolves when we have the records
Added `load` to a couple more tests.

We should verify that this is the right API before doing this too much maybe;

foo.get('comments').then => semantics of this promise?

- comment ids are loaded
  (and comments loaded lazily via objectAt)
- comments are loaded
Merge pull request #1853 from hjdivad/single-source-of-truth

some ssot work with @igort