CAF::Render to render structured text

Make these a little bit more Perl-ish:

```perl
$self->{log} = $opts{log} || CAF::DummyLogger->new();
$self->{templatebase} = $opts{templatebase} || $DEFAULT_TEMPLATE_BASE;
$self->{relpath} = $opts{relpath} || $DEFAULT_RELPATH;
```
since auto-vivification is not a problem here.