Added Android-PullToRefresh support to subreddit list.

1. Added Android-PullToRefresh support to subreddit list. Pull the list down to refresh it.
2. Subreddit list enhancements which make it much faster. Now it won't query the list from reddit.com every time. So it feels super snappy.

Since I am adding a new submodule to Diode, I am not super sure if this won't create any trouble in development environment of others.

You don't have to pull right away. Clone my repo, check it out if it would work fine in your dev environment.

If there are build issues, you can try the apk meanwhile: https://docs.google.com/file/d/0B4VtWtp1e2j5YWtXMUhKUmdqUFE/edit?usp=sharing