Added schema level support for publication.

Hi,

This feature adds schema option while creating publication. Users will
be able to specify one or more schemas while creating publication,
when the user specifies schema option, then the data changes for the
tables present in the schema specified by the user will be replicated
to the subscriber. Few examples have been listed below:

Create a publication that publishes all changes for all the tables
present in production schema:
CREATE PUBLICATION production_publication FOR ALL TABLES SCHEMA production;

Create a publication that publishes all changes for all the tables
present in marketing and sales schemas:
CREATE PUBLICATION sales_publication FOR ALL TABLES SCHEMA marketing, sales;

Add some schemas to the publication:
ALTER PUBLICATION sales_publication ADD SCHEMA marketing_june, sales_june;

Drop some schema from the publication:
ALTER PUBLICATION production_quarterly_publication DROP SCHEMA production_july;

Attached is a POC patch for the same. I felt this feature would be quite useful.
Thoughts?

Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com