Stop mentioning "previous" snapshot

Now that we've moved over to comparing with a baseline, it doesn't make
sense to talk about "previous" snapshot. This commit updates copy and
changes a method name.
Add section about running the server to README
Align code in Url model
Remove misleading comments in UrlsController
Remove misleading comment in SnapshotsController
Align code in SnapshotsController
Re-wrap application JS/CSS comments at 80 characters

Change-Id: Ib4292d762958cf4bdac96b98475dfa93076120e2