Classfiy custom types and add EnumType to it.

I added the file `sqltypes.py` to classify custom types in SQLAlchemy. The classes in the file should inherit `sqlalchemy.types.TypeDecorator`. Also, I added `EnumType` to the file. It makes a enum type in SQLAlchemy be used as Python `enum.Enum`.
[Add the following method:](http://permalink.gmane.org/gmane.comp.python.sqlalchemy.user/36710)

```python
def _set_parent(self, column):
    self.impl._set_parent(column)
```