* Fix some namespace issues, previously wasn't qualifying embedded message.

* Switched from template (compile-time) to weak link (link-time) Check
  function definition. This allows us to resolve #39.

* To support the above, switched away from inlining and header only
  generation for C++ to a split header/implementation structure.

Fixes #39.

Signed-off-by: Harvey Tuch <htuch@google.com>