Allow PageImages to be used out of the Blog / Page context.

I wanted a way to use PageImages in my custom engines, so I went ahead and changed the code a bit.

The javascript could use a bit of love, it feels kinda messy out of the original context, but at least it works. I may improve it at a later time.

 Here's exactly what I did :

- Explicit association classes so the extension works in non-Refinery-namespace engines;
- Add an option to disable inclusion in Page or Blog extensions.

What do you think?