Support Immutables in records.

[lukesneeringer]

This PR adds support for the use of immutables in records:

import {Record} from 'typed-immutable';

var MyRecord = Record({
  mapName: Immutable.Map,
});

The object form (e.g. new Immutable.Map()) also works, if you wish to provide a default.

This allows the original Immutable objects to be used within typed-immutable records when appropriate. The above would roughly be equivalent to Map(Any, Any), and Immutable.List would be equivalent to List(Any); however, we will no longer incur the overhead of using our own objects in these cases when we do not need the typing system.

Additionally, any additional objects Immutable provides, now or in the future, work. So, for instance, this supports Immutable.Set.