add search_path attribute to postgresql_psql resource

This allows you to set a schema search_path on postgresql_psql
resources, in case you have multiple schemas in your database and the
SQL you are trying to run requires a different path
postgis, try #1
updated README and tests for postgis
periods are valid in configuration variables also

example:
http://www.postgresql.org/docs/9.2/static/auto-explain.html