Add application setting to disable ETag (again)

It's updated to upstream duplicate of the old Pull Request #1421.

Old Pull Request description:

> Add application setting app.disable('etag'); to disable ETag. Manual ETag addition via res.set('etag', 'somestring);' still works.
> 
> Please, check this feature.
> 
> Btw, it's my first experience in Express and JS BDD, so, please, if there are any issues or mistakes (for ex., I am not sure about proper spec placement), let me know, maybe I'll can fix 'em.
> 
> Thanks!

@visionmedia, sorry for such a long delay, I totally forgot about this pull request and that I need to sync it with upstream. I hope everything is good now.