Extract template related object store features

This moves all template-specific object store features into another module monkey patch stored in lib/templates.rb, and moves the related tests as well.

Once again, this probably makes Prawn's code simpler while making the template code even more brittle, but at this point I think the best person to improve the stability of this code would be whoever takes over prawn-templates, rather than me. I'd be happy to add extension points to make this more stable, but I don't want to do the research myself on that for a feature that will be unsupported indefinitely.

Anyone with major reservations, please speak up in the next couple days. Otherwise, I'll merge this code and include it in 0.14.1 to kick the tires, and we'll plan to remove it entirely from Prawn by 0.15.0.