Don't convert underscores in namespaces to directory separators

PSR-0 states that underscores in namespaces have no special meaning.

This was mapping class names such as
```
name_space\Class_Name
```
to:
```
name/space/Class/Name.tpl.php
```

when it should map to 
```
name_space/Class/Name.tpl.php
```

PSR-0 source: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md