Snowconn

New features:
- You can now specify a role in the connect call.

Issues fixed:
- Fixed the read_df call.

Included wheel as the first dependency in setup.py because there are some other dependencies that are throwing errors if it is not there. most of the time it does not seem to make any difference but with some of Louis' deployments it's causing problems.

There is now a new `temporary_table` parameter to the `write_df` function.

`write_df` was using the sqlalchemy engine before when it should have been using the connection. this is what was causing at least some of the rollback problems.

only using engine.dispose() because it still generated undesired rollbacks in a few situations

stopping last statement from being rolled back automatically because of incredibly stupid snowflake-connector behavior

if a json deserializeable type was null would still try to call json.loads which would throw an error. fixed now.

not documented though. not sure if it's actually useful to anyone but sam