Don't display captions when they're empty (v0.2.1)

This fixes some very minor whitespace collapsing issues with figcaption.
Merge branch 'master' of github.com:PiPeep/pan-am
Greatly improve Makefile

- Only recompile the tests that have changed (Using macros and
  auto-generated targets! Eat that, automake!)
- Compute file size and gziped size and display them
- Use the `.PHONY` directive where relevant
- Hide some minor commands and write out others clearer
Add a slight background noise (v0.2.2)

The background noise makes the document look more like it's written on
paper.

The noise images are provided by the noisy-uris project:
https://github.com/markdurrant/noisy-uris

To avoid making another HTTP request, the images are base64 encoded, and
`@extend` is used to avoid duplicating the images.