Make it possible to add non-theme default templates

[voxpelli]

To get more in line with the design of the Drupal theme system (which node-tema is heavily inspired by) it should be possible for non-themes (similar to Drupal modules) to provide default templates so that one don't have to create a base theme to contain all such basic templates and have all new themes inherit from that base theme. If non-theme templates can be added then new themes can much more easily be added without the need for inheriting from a base theme.