move schema creation to after hooks

[drewbanin]

Fixes #604

This PR moves schema creation to after the on-run-start hooks. This makes it possible to add a hook to resume a Snowflake warehouse, for instance. This hook could look like:

on-run-start:
  - alter warehouse {{ target.warehouse }} resume;

Note: This could break existing projects if there is code in the on-run-start hooks that expects the dbt schema to exist. In this case, a line can be inserted into the on-run-start section to create the schema.