Fix Unique validator's session check

Consider the following module:

```python
from wtforms_components import ModelForm, Unique

class UserForm(ModelForm):
    name = TextField()
    email = TextField(validators=[
        Unique(
            User.email,
            get_session=lambda: db.session
        )
    ])
```

where `db` is an instance of `flask.ext.sqlalchemy:SQLAlchemy`.
Attempting to just import the module previously raised `RuntimeError`
because session was checked in `Unique.__init__`:

```pytb
user_form.py:3: in <module>
    class UserForm(ModelForm):
user_form.py:8: in UserForm
    get_session=lambda: db.session
env/lib/python3.4/site-packages/wtforms_components/validators.py:209: in __init__
    self._check_for_session(self.column.class_)
env/lib/python3.4/site-packages/wtforms_components/validators.py:225: in _check_for_session
    if not hasattr(model, 'query') and not self.get_session:
env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py:428: in __get__
    return type.query_class(mapper, session=self.sa.session())
env/lib/python3.4/site-packages/sqlalchemy/orm/scoping.py:71: in __call__
    return self.registry()
env/lib/python3.4/site-packages/sqlalchemy/util/_collections.py:910: in __call__
    return self.registry.setdefault(key, self.createfunc())
env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py:136: in __init__
    self.app = db.get_app()
env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py:809: in get_app
    raise RuntimeError('application not registered on db '
E   RuntimeError: application not registered on db instance and no application bound to current context
```

I removed the session check from `__init__`. Now it's checked when
the `Unique` validator is actually called.