Revisit materialized views

[jtcohen6]

Substantive changes

• Resolve Snowflake materialized view fails on 2nd run #14 — Snowflake's caching logic can't differentiate between materialized views and views, so we just need to document that switching between view and materialized_view is unsupported
• Fix auto_refresh logic on Redshift
• Stick dispatched macros in the dbt macro namespace, rather than dbt_labs_materialized_views — this is a judgment call, to more closely mock the experience of these macros living "inside" dbt-core

etc

• Revise README, including recommendations for git + subdirectory package installation and macro dispatch
• Upgrade dbt-utils dependency