Install `.mli` files with the library binaries

closes #11
Use a Uri.t in Atom instead of just string; discussed with @samoht

Now depends on Uri library
add `Atom.mk_link` helper function
Document the `-cow-no-open` camlp4 flag

Closes #9
correct META file dependencies
Be compatible with Core by using `List.concat` instead of `List.flatten`

Closes #9