Support for COMMIT_TIMESTAMP

`spanner.COMMIT_TIMESTAMP` is a special value that may be used for automatically setting timestamp fields using commit time. However, the ORM doesn't support this:
```
/usr/local/lib/python3.7/site-packages/spanner_orm/model.py in __init__(self, values, persisted)
462
463 for column in self._columns:
--> 464 self._metaclass.validate_value(column, values.get(column), ValueError)
465
466 for column in self._columns:

/usr/local/lib/python3.7/site-packages/spanner_orm/model.py in validate_value(cls, field_name, value, error_type)
125 cls.fields[field_name].validate(value)
126 except error.ValidationError as ex:
--> 127 raise error_type(*ex.args)
128
129

ValueError: spanner.commit_timestamp() is not of type datetime
```