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
Traceback (most recent call last):
  File "user_form.py", line 3, in <module>
    class UserForm(ModelForm):
  File "user_form.py", line 8, in UserEditForm
    get_session=lambda: db.session
  File "env/lib/python3.4/site-packages/wtforms_components/validators.py", line 209, in __init__
    self._check_for_session(self.column.class_)
  File "env/lib/python3.4/site-packages/wtforms_components/validators.py", line 225, in _check_for_session
    if not hasattr(model, 'query') and not self.get_session:
  File "env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py", line 428, in __get__
    return type.query_class(mapper, session=self.sa.session())
  File "env/lib/python3.4/site-packages/sqlalchemy/orm/scoping.py", line 71, in __call__
    return self.registry()
  File "env/lib/python3.4/site-packages/sqlalchemy/util/_collections.py", line 910, in __call__
    return self.registry.setdefault(key, self.createfunc())
  File "env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py", line 136, in __init__
    self.app = db.get_app()
  File "env/lib/python3.4/site-packages/flask_sqlalchemy/__init__.py", line 809, in get_app
    raise RuntimeError('application not registered on db '
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.