m2m relation population

I've made rough solution for population of m2m using model_mommy, this is very useful in real life when you are dealing with django.contrib.auth.models.User model

Currently, to test authorization limitation in my API I have to write something like:
```python
    user = mommy_recipes.user.make(
         groups=[mommy_recipes.public_api_group.make()]
    )
```
I'd rather do something like:
```python
    user = mommy_recipes.public_api_user.make()
```
So, with this branch, the recipe will be:
```python
    public_api_user = Recipe(
         am.User,
         groups=related(public_api_group)
    )
```
And everything should work fine.

Honestly while I was writing this, I've realised that I didn't do proper testing. So I'll extend my test a bit.