Switch to adding new entries unobtrusively

See #62 for context.

Here we:
* Add new entries automatically if we're on top
* Show a fixed nag with the # of waiting entries,
otherwise.
* Start including some Backbone magic to untangle the
callback mess. See #70.
* Remove old nag code.

We still aren't:
* Handling updates and deletes separately
* Having the best fixed nag behaviour
Introduce Entry::type

It can be one of new, update, delete.

This way we won't need to distinguish the 3 by
heuristics like whether there is any content, or
whether the node already exists.
Take advantage of the entry type in the JS side
Don't queue update/delete entries

Updating and deleting entries is rare enough, so that
we can screw the user's scroll and not queue those events.
Use only CSS selector when defining view elements

We don't need to supply a jQuery object.
Move fixed nag to the top

This way it will be as theme agnostic as possible.

If there is an admin bar, we push it right below
the admin bar.

The behaviour didn't change – clicking it will
flush the queue and take us the the top of the page.
Vertically align the updates number
Get translated nag text from the server side
Give the large number some space
Apply modifying entries to the queue

Imagine the following scenario:
 * An editor posts an entry
 * The user gets a nag with 1 new entry
 * The editor deletes the just-added entry
 * The entry is deleted, but since the queue isn't
touched, the user still sees the same message.

In this commit we apply all new modifying (update/delete)
entries to the queue. If we get a delete entry for an
entry in the queue, we remove it from the queue. If we
get an update entry, we just update the html of the
entry in the queue. Both of these trigger a re-render
of the fixed nag and it's always correct.
Merge pull request #72 from Automattic/feature/unobtrusive-updates

Unobtrusive automatic entry updates