Use Omd consistently; move tests outside src/
Split omd.ml in an implementation and interface file.

Get rid of the unnecessary Md sub-module.  Name the main type [t] as it
is customary.
Conform to 80 chars width, remove trailing white space.

Some constructions were also changed to be more standard and code
re-indented in some places.
Generate .annot files during compilation.
Omd.to_html is more natural than Omd.html_of_md
Remove the unecessary constructor [Li].
Use List.exists instead of a personal version.
Relicense under ISC.
Add replace section in _tags
Enable tests (compilation + oasis + make).
Report the number of tests that passed and failed.
Merge pull request #5 from Chris00/cleaner

Improve code quality.
That was a little early but ok.