Supply additional parameters to create_engine function

[cheungpat]

py-skygear provides function to create a SQLAlchemy engine connection, which calls create_engine() with the database URL. There is a request to set additional parameter for the create_engine() function, such as pool_pre_ping. This can be achieved by modifying the conn function (in skygear/utils/db.py) to pass kwargs to _get_engine() and create_engine().