- get the "stamp" command to work in as_sql

- get the "stamp" command to work in as_sql
- overhaul context + env so that --sql mode truly does
not make any SQL connections of any kind.   The env.py
scripts create the engine and use it as the source of
a "dialect" - the "dialect" is now passed straight to the context.
- more power to env - can set output buffer, transactional ddl flag,
execute SQL via context instead of needing to import op