Flexible template folder schema

I needed a template directory schema where the flavor prefix cames after the last slash, like in: `/appname/{flavor}-template_name.html`

Then created the `FLAVOURS_TEMPLATE_NAMEGETTER`, to be able to have such schema. Example:

```python
# settings.py
def _mobile_template_getter(flavour, template_name):
    folder, sep, filename = template_name.rpartition('/')
    return u'{folder}{sep}{flavour}-{filename}'.format(**locals())

FLAVOURS_TEMPLATE_NAMEGETTER = _mobile_template_getter
```

What you think?