tests: Restructure error tests some more
tests: Restructure normal tests too
design.txt: Clarify & rewrite

* It's about design decisions, and does currently not contain much info on
  fbfrog internals
* Some parts rewritten to make the point from fbfrog's POV, instead of the
  opposite.
Move existing presets into contrib/

because now they're just "contributions" that may be helpful to others
using fbfrog, but not built-in presets.
contrib: Adjust existing presets for recent changes
contrib: libbig_int preset
contrib: libclang preset
contrib: CUnit preset
cli: Accept *.fbfrog as @files too
Improve --help output and readme.txt

* Better formatting
* @file and *.fbfrog documentation was missing
Set version to 1.0

The new fbfrog is really shaping up and coming together now... and it's
extremely different from 0.1 and the unreleased 0.2.