Implement selection of which templates to serve from S3

[thiagofigueiro]

This allows you to do something like this in s3_rails.yml:

  include_list: ['reports/yearly_report.pdf.prawn', 'reports/monthly_report.pdf.prawn']

It's a fairly quick-and-dirty implementation that I did on a pinch to get me going with a project at work.

If someone has some time it would be nice to:

• Add tests.
• Implement the filter when fetching S3 objects (load_cache) instead of when looking-up cache (query).

[thiagofigueiro]

Hey, @straydogstudio - sorry about that dud PR yesterday.

This one is working as expected, even thought I'm not super confident in how I implemented it. In any case, it's here for using or throwing away as you see fit.

Cheers