Disallow live updates when Heartbeat is missing

Resolves #599

Rather than using the `wpe_heartbeat_allowed_pages` filter, I opted for a simpler and host-agnostic approach instead.

1. The javascript code for live updates now has its own handle `wp-stream-live-updates`
2. Because of this, the `wp-stream-admin` JS no longer depends on `heartbeat` to be enqueued.
3. The `wp-stream-live-updates` JS _does_ depend on `heartbeat` to be enqueued.
4. If `heartbeat` is not loaded, and the user tries to enable live updates, they will receive an alert explaining that the operation cannot be performed and to contact their hosting provider.

![screen shot 2014-07-29 at 1 56 52 pm](https://cloud.githubusercontent.com/assets/522158/3740146/26978784-1752-11e4-9491-15e05b069134.png)

The end result for the user is that they can then choose whether or not they want to use something like the [WPE Allow Heartbeat](https://github.com/PrysPlugins/WPE-Allow-Heartbeat) plugin instead of us making that decision for them. This could also be a trend other hosts follow in the future, so handling it this way is more future-proof.

/cc @lukecarbis Please review