Switch to single-storage of feeds

Now a feed an it's entries are only fetched once. This has a major advantage
in regards of storage. Next to the this decouples fetching the feeds from
the page reload which heavily reduces the load time.

This currently is a work in progress so don't expect everything to work.
Format the code and organize imports
Improve the DAOs by establishing a filter concept

Basically this boils down wrapping the criteria API in something more
convenient.
Move more DAO functionality to filters
Switch all DAOs to filters
Improve the happiness of our unit tests
Drop JPA in favor of direct datastore access

As sad as it is, but the JPA implementation of AppEngine hurts more
than it helps. Or at least it behaves very different to what I'm known
from Hibernate. Let's see if direct access works better.