The TemplateLoaderMixin class is a simplistic loader which performs no caching.
| base_dir) | 
| name) | 
os.path.join() on the base_dir specified in the
constructor and the name argument.
If there is an error reading the template a TemplateLoadError will be raised.
The class remembers the names of all loaded templates.
| name) | 
None if the template specified in the name
argument has been previously loaded.  If not previously loaded it is
loaded via the load_template() method and returned.