Move static flash message methods out of Luthier_Core

[vimofthevine]

The static methods for setting and retrieving flash methods should not be a part of Luthier_Core, but rather Luthier_Message. This allows for an extension to replace Luthier_Message and implement a different message/notification solution without interfering with the core helper (though...there aren't any actual methods in it...).

This is related to the issue of multiple inheritance between modules. In Luthier, it is preferred to have many small helpers rather than a single core helper (i.e., Luthier_Acl, Luthier_Auth, etc.).