designs/spammer/README.md: Documentation for the Spammer app.

This is an idea for how designs could be documented in general.
designs/spammer/README.md: Added git commit to performance table.
(Minor formatting fix.)
Generate inline "ditaa" diagrams in README.md files.

Now README.md.src files can use an extended markdown syntax:

    DIAGRAM: foo
    ... multi-line ascii-art ...

which uses ditaa (http://ditaa.sourceforge.net/) to create a PNG
image from the ascii art and put an inline image link in the
outpt markdown.

The files are output to README.md and should look pretty on github.
Added src/apps/README.md with diagrams.
process-markdown: Store images in .images/ to avoid clutter.
Refactored from core.app into apps.basic and designs.basic.

The basic apps Source, Sink, etc, are now in apps.basic.basic_apps.

The basic test app design is now in designs.basic.basic.