Fix comments to indicate that this program is not Emacs

Old comment block was a copy from Emacs.  It said this program is GNU
Emacs, but obviously not.
Add test code for ox-asciidoc

ox-asciidoc is still small piece of code but it already passed my
ability to check all of the combination of transcode pattern with
local test files. ERT is much easier to cover behaviors of the
ox-asciidoc.el compare to those test files.

This is a first step toward TDD with ERT.