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`.
Trivial grammatical changes needed:

- The trailing period (`.`) is missing.
- It would better if `enum.Enum` is actually linked to the corresponding reference docs.  You can use `:class:` role here e.g. ```:class:`enum.Enum` ```.