Asyncpgsa

* Adds support for custom column types (see https://github.com/CanopyTax/asyncpgsa/pull/42)

Shoutout to achimnol for implementing this.

* bugfix, `pg.execute()` now maintains args when passing a string. See https://github.com/CanopyTax/asyncpgsa/pull/39. Shoutout to fantix for the fix.

changes
1. Changed the dialect from psycopg2 to pypostgres. This should be
mostly backwards compatible, but if you notice weird issues, this is why.
2. You can now plug-in your own dialect using `pg.init(..., dialect=my_dialect)`,
or setting the dialect on the pool. See the top of the connection file
for an example of creating a dialect. Please let me know if the change from
psycopg2 to pypostgres broke you. If this happens enough,
I might make psycopg2 the default.