add support for custom sqlalchemy dialects in create table statements

Created by: pedros

Currently, creating table statements is limited to SQLAlchemy's built-in dialects. This patch substitutes loading dialects via sqlalchemy.dialects.registry for __importing__ from sqlalchemy.dialects.name. This allows for loading dialects added via setuptools entry_points. For example pip install pyhive, which I tested with, provides SQLAlchemy dialects for Hive and Presto.