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

- Explicit association classes so the extension works in
non-Refinery-namespace engines;
- Add an option to disable inclusion in Page or Blog extensions.
Initialize tabs and attach models using a generalized logic.
Add test coverage
Change initializer for a #to_prepare block.

This ensures refinery-page-images gets attached
on each request in development. No more `undefined method images`.
Fix specs under postgres
Merge pull request #81 from hooktstudios/master

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