Issue #207: some minor refactoring

I want to rename some classes, so I'm just checking in some small
changes here. Nothing noteworthy...
Issue #207: rename some adapter classes

Rename ThingAdapter to ThingListAdapter, because we want to create a
class called ThingAdapter for ThingFragment. Rename
SubredditListingAdapter to SubredditListAdapter to follow the convention
too.
Issue #205: allow subreddits to be dismissable

Even if you don't have an account, you should be able to delete
locally stored subreddits.
Issue #205: define separate table for comments

Use a separate table to scope deletions when hiding easier.
Issue #205: create the comments table
Issue #205: add base thing columns to some tables

The comments table has to implement BaseThingColumns, because we want
the user to be able to save or hide via the header comment.
Issue #205: use comments table in CommentListing

Also add saved column to BaseThingColumns to track the remote saved
state of a thing when either hiding or saving it.