Simplify some Map and Set signatures.
Merge pull request #139 from Drup/map_sig

Simplify some Map and Set signatures.
Merge branch 'master' of github.com:ocaml/omd
fix: add a %! to flush warnings as they are issued

modified:   omd_utils.ml
feature: avoid printing \n\n if nothing follows

modified:   omd_backend.ml

might resolve https://github.com/ocaml/omd/issues/138#issuecomment-61417664
fix issue with HTML and newlines in Markdown output

Should fix https://github.com/ocaml/omd/issues/138#issuecomment-61417664

modified:   omd_backend.ml
modified:   omd_main.ml --> make sure the output ends with a `\n`, which is necessary following the modifications in omd_backend.ml