Dbt-synapse

- pull AD auth directly from `dbt-sqlserver` (https://github.com/swanderz/dbt-synapse/pull/13)
- hotfix for broken `create_view()` macro (https://github.com/swanderz/dbt-synapse/pull/14)
- get `dbt-adapter-tests` up and running (https://github.com/swanderz/dbt-synapse/pull/16)
- make `sqlserver__drop_schema()` also drop all tables and views associated with schema
- introduce `sqlserver__get_columns_in_query()` for use with testing
- align macro args with `dbt-base`

New Features:
- Adds support for dbt v0.18.0

- Add CI testing (19)
- Remove the external table macros in favor of pulling them directly from `dbt-external-tables`
- Bundle the "`INSERT` & `UPDATE`" `MERGE` workaround into a transaction that can be rolled back (23)
- Handle nulls in csv file for seeds (20)
- Verifed that adapter works with `dbt` version `v0.18.1`

Fixes:
- added snapshot functionality

Fixes:
- initial release