Refactor PgView to use PgColumn for storage of column declaration list, defaults and comments
Initial support for materialized views (PostgreSQL 9.3+)
Unify Alter{View,Table}Parser into AlterRelationParser, implement ALTER MATERIALIZED VIEW
Unify ALTER TABLE ... ALTER COLUMN code for views & tables
Unify COMMENT ON COLUMN parsing between tables and views
Add PgRelation.getRelationKind to distinguish VIEWs from MATERIALIZED VIEWs, unify SQL generation for DROP and COMMENT statements