Visibility header should be distributed

This header must be distributed since it is included by practically all
other headers in the module!
Symbol must be visible also in the imported module

Even though we won't export it further, marking it as hidden will make
it disappear from view even in the module it is declared (if it is not
defined there). The distinction between visible/hidden is different from
that of dllexport/dllimport in the Windows-world.
Merge pull request #6 from rolk/006_visibility

Make public classes visible at the import site