Add feed.json file

This file contains pre-downloaded iOS related feed
from various sources. By reading feeds directly from
this file, it will be easier for us to test the rest of the logic
in the app.
Fix a typo in Feed entity name
Use feed list nav controller as window's root VC
Center feed image to avoid stretching
Provide a way to retrieve all feed from database
If the builder doesn't return any feed, retrieve from db instead

The builder will only create new feed object for the ones that
don't already exist in database. Therefore, we need to fetch
directly from database and return it to the delegate if that
happens.