schema_search_path: An optional schema search path for the connection given as a string of comma-separated schema names. This is backward-compatible with the :schema_order option.
encoding:           An optional client encoding that is used in a SET client_encoding TO <encoding> call on the connection.
min_messages:       An optional client min messages that is used in a SET client_min_messages TO <min_messages> call on the connection.
variables:          An optional hash of additional parameters that will be used in SET SESSION key = val calls on the connection.
insert_returning:   An optional boolean to control the use or RETURNING for INSERT statements defaults to true.

Any further options are used as connection parameters to libpq. See www.postgresql.org/docs/9.1/static/libpq-connect.html for the list of parameters.
In addition, default connection parameters of libpq can be set per environment variables. See www.postgresql.org/docs/9.1/static/libpq-envars.html .