Let's change a few function names:
- ``set_cache`` to just ``set``
- ``get_cache`` to ``get`` (this clashes with from django.core.cache import get_cache in https://github.com/ui/django-post_office/pull/2/files#L3R1 too)
- ``delete_cache`` to ``delete``

We can then use these APIs as such:

```python
from post_office import cache

cache.set('template_name', 'content')
cache.get('template_name')
cache.delete('template_name')
```