Registering rules through decorators

Added two decorators -- `@rule` which is meant for functions and `@rules` meant for classes -- to ease in registering rules.

When declaring permission rules for a model, decorate the model with `@rules()` to auto-register all `can_FOO` functions as permission rules for that model. Decorate the model with `@rules('can_foo', 'can_bar', '...')` to be more explicit if desired.

```python
import rulez

@rulez.rules()
class Foo(models.Model):
    def can_read(self, user):
        return False

# or to be explicit ...

@rulez.rules('can_read')
class Bar(models.Model):
    def can_read(self, user):
        return False
```

When declaring a permission rule outside of the model, decorate with `@rule(codename, cls)` as follows:

```python
from django.contrib.auth.models import User
import rulez

@rulez.rule('can_read', User)
def user_can_read(obj, user):
    return False
```

I believe it makes declaring rules a bit cleaner and more pythonic. I tentatively placed the decorators in `rulez/__init__.py`. Opinions?