Add support for SQAlchemy Enum field

If you have code::

```python

class Category(Model):
    status = Column(Enum('category_status1', 'category_status2'), name='statuses')

class Product(Model):
    status = Column(Enum('status1', 'status2'), name='statuses')
    category_id = Column(ForeignKey(Category.id))
    category = relationship(Category)

class ProductView(ModelView):
    column_labels = form_labels = {
         'status': u'Статус продукта',
         'category.status': u'Статус категории',
    }
    column_choices = form_choices = {
        'status': [
             ('status1', u'Статус продукта 1'), 
             ('status2', u'Статус продукта 2'), 
        ],
        'category.status': [
             ('category_status1', u'Статус категории 1'), 
             ('category_status2', u'Статус категории 2'), 
        ],
    }
    column_filters = [
        'status',
        'category.status',
    ]
   
```

then you will have automatically:
1. Correct and readable SelectField for `status` field
2. Readable values in table for columns `status`, `category.status`
3. Choice filters