Adds support for pushing promise objects to hasMany relationships

 Added the hasMany support further out as you suggested. Has a few tests for the various code-paths, not sure of the best way to test that all of them work as expected... Hard when theres like 10 ways to add a record to a `ManyArray` :(

@igorT 
I think this works by accident, due to how our testing framework is set up. In real world, only things passed from relationship 'get' would work, not things passed from store.find.