Support webserver cache invalidation

[pestevez]

We should add some way to support invalidating the cache that the webserver has (when used.)

There is one scenario that I think is the most important which is when the HTML page used to create the template has changed, which will prevent us from getting the correct preview and source views, while the browser window may be displaying the updated version.

Memcached has multiple settings to control the time to live of a the cached items, so I am not as concerned about allocated memory or staled items, but it is something that also should be analyzed.