Closes #52 by loading a models hasMany relationships before saving

This Pull Request Closes #52.  

The problem was that the JSONSerializer's default implementation of `serialize()` didn't return hasMany associations.  So when `_saveHasManyRelationship` was detecting records that were removed from a hasMany association it flagged all of the associated records because they didn't appear in the `ids` passed in from `updateRecord`.

I changed the implementation of `_getSerializedRecord` to include the async hasMany association ids in an array on the json object.  I also had to slightly change the implementation of `updateRecord` to deal with the promise returned by `_getSerializedRecord`