Fix if_exists and on_cluster AttributeError-s

[vmarkovtsev]

This PR makes the following code execute without errors:

import sqlalchemy
import clickhouse_sqlalchemy.engines

metadata = sqlalchemy.MetaData()

notes = sqlalchemy.Table(
    "notes",
    metadata,
    sqlalchemy.Column("id", sqlalchemy.Integer, primary_key=True),
    sqlalchemy.Column("text", sqlalchemy.String(length=100)),
    clickhouse_sqlalchemy.engines.Memory(),
)

engine = sqlalchemy.create_engine("clickhouse://default:@0.0.0.0:8123")
metadata.create_all(engine)
metadata.drop_all(engine)

Related to #22

Can you please make a new PyPI release after merging this.

[coveralls]

Coverage remained the same at 95.226% when pulling 2b20f32 on vmarkovtsev:fix-drop into f3df5f8 on xzkostyan:master.

[xzkostyan]

Sure. Version 0.1.4 is released.