Add quarter time grain to postgresql (#1362)

The timestamp returned is the start date of the period.

Reference:
https://www.postgresql.org/docs/9.1/static/functions-datetime.html

caravel/db_engine_specs.py

@@ -56,6 +56,7 @@ class PostgresEngineSpec(BaseEngineSpec):
         Grain("day", _('day'), "DATE_TRUNC('day', {col})"),
         Grain("week", _('week'), "DATE_TRUNC('week', {col})"),
         Grain("month", _('month'), "DATE_TRUNC('month', {col})"),
+        Grain("quarter", _('quarter'), "DATE_TRUNC('quarter', {col})"),
         Grain("year", _('year'), "DATE_TRUNC('year', {col})"),
     )