added imports on structure docs

[leportella]

I was looking the docs and this example had no imports and the final loop. Since I'm new on tornado, I thought that it would be better to have this so beginners such as me wouldn't be lost. I added the imports and the app loop.

Also, since there is no really use for the db, I would suggest to take it off, to make it workable with a simple copy-and-paste. However, since I didnt't know the purpose of this, I didn't take the db off it.

[bdarnell]

The db object isn't used here, but it does have a point: this example is also meant to show one way to handle database connections and similar global state in Tornado applications (and the third argument to the URL spec).

Even with these changes, the example isn't copy/pasteable because of the missing db object (also the indentation looks wrong, but that may just be github confusing me). Maybe what's needed here is two separate examples: one copy/pasteable example showing the use of capturing groups for the story_id argument, and a separate one (maybe copy/pasteable, maybe not) showing the db setup.

The blog demo is a more complete database-using example; maybe we should just point there (although it doesn't actually use the third URLSpec argument, and instead stuffs the database in the application). Maybe a better example of that third argument is RedirectHandler.