Make dir relative to page and auto-generate thumbnails

[awein]

Thanks for this great project!

I would like to suggest the following changes:

• Use page resources to organize is really nice and clean. That's why dir is relative to the page directory now.
• Automatic creation of thumbnails makes this even more nice and easy thanks to the image processing
  feature.

[liwenyip]

Hi @awein

Thanks for your contribution.

I'm definitely keen to adopt page resources, as it simplifies a lot of things; but I want to have a transition where users can choose to create galleries the old way using images in static or the new way using page resources.

This a duplicate of #46 so I'm going to close this PR for now.

Thanks,
Li-Wen