Solution: regenerate to fix paragraphs headers

Solution: regenerate it

Fix Issue #122: Add short description gsl and gsl acronym to README.txt and regenerate README.md using gitdown.

Solution: edited README.txt to add .terminator example and
	regenerated README.md using this step:
```
$ gitdown README.txt
```

Solution: Regenerate it

Solution: Fix by adding newline

Solution: Fix with semicolon

Solution: Add support for Cygwin

Solution: Regenerate using new gitdown, which makes a cleaner TOC

...and not obvious that it is even generated

Solution: regenerate README.md, as gitdown now adds a line saying that the document was generated by gitdown

It's either ugly if written as $(...) (note that `...` can't be used as it
will show $ literally, not replace it with a dollar-sign), or it
causes gitdown to issue errors if written as `$(...)`.

Solution: use <tt>&#36;(...)</tt> trick

Apparently, this still replaces the HTML escape sequence with the
dollar-sign, but is still "invisible" to gitdown, so it won't issue any
errors about undefined variables.

Documentation got camelCase wrong (was CamelCase).
PascalCase actually exist in code but not in documentation.

- to modify escape char from \
- to modify substitute sequence from $(
- only affects scripts loaded after this is changed
- use -escape:x and/or -substitute:xx from command line
- see testesc.gsl

Added 'code' and 'headers' targets into Makefile

- lets us regenerate all GXL code
- lets us rebuild the gsl.h file, since we don't use Boom any more

Updated directory.open to read all directory contents at start
rather than during iteration in order to avoid any file
problems during iteration such as unreadable files or dangling
symlinks, etc.

See https://github.com/imatix/gsl/issue/12