Fix template names when walkdir returns absolute paths.

I used the following command in Ubuntu 12.04.2 with node 0.10.12

```bash
haml-coffee -i . -o ../some/path/templates.js
```

The template names contained absolute paths instead of relative paths. I traced that to walkdir returning absolute paths when called as `walkdir.sync('.')`. This change works around the `walkdir` oddness and gets the correct template names.