Preview server now polls for fresh content

It takes a while (up to 20 seconds) for updates to be reflected in CAPI. This causes problems while previewing content as you are not sure what has taken effect and what has not.

Working with composer, the preview server (*only preview*) will now understand that the content being viewed is stale, display a message that it is waiting for an update, and then refresh once the update is available.

This will only work if an expected `last-modified` date is attacjed to the URL in the form of a hash (composer preview button will do this)...

`/uk-news/2014/oct/03/alan-henning-isis-syria-video-murder#last-modified=2014-10-10T05%3A04%3A30%2B01%3A00`

![polling](https://cloud.githubusercontent.com/assets/76709/4578997/06e6c6b6-4fc9-11e4-8f8d-73aac7dd555f.png)

Rather than having to pass in a `block` callback function, it would be nicer to just return the promise from this function.