Added a 'DogpileAction'.

This is designed to soak up large numbers of hits on a single page (e.g. end of live football match, election of Pope). If the application is currently fetching and rendering that page, then this action will await that result instead of duplicating the effort.

Here is a graph of me putting this under 'siege' on my local machine...
![dogpile](https://f.cloud.github.com/assets/76709/2060209/a79a8fdc-8c03-11e3-84cb-d979966bf131.png)