16 add new user log user

close #16
możesz użyć [min length validator](https://docs.djangoproject.com/en/1.7/ref/validators/#minlengthvalidator)

```python
from django.core import validators

password = forms.CharField(
    label='hasło', 
    widget=forms.PasswordInput,
    validators=[validators.MinLengthValidator(6)],
    error_messages={
        'min_length': u'Hasło musi mieć co najmniej 6 znaków'}
)
```

tak, jak tutaj: https://github.com/v0y/sport-tracker-with-acziwments/blob/master/app/accounts/forms.py#L131-L137