Add auto-screenshot option

[jqnatividad]

Showcasing open data in action almost always requires an image to be compelling. If one is not specified, present an option to create an auto-webshot using the URL with something like http://webshot.okfnlabs.org/.

It can even be something the showcase_admin desires - i.e. use the latest screenshot of a highly dynamic site that's updated on a showcase_admin-specified interval (e.g. 1x day/week/month)