Added object_exists_query/2 to Postgres.SQL
Added `object_exists?/2` to Adapter
Added `exists?/2` to Migration.Runner
Added `column_exists?/2` to Migration.DSL
Added `table_exists?/1` to Migration.DSL
Updated Postgres.SQL.object_exists_query to use pg_* instead of information_schema
Added `index_exists?/1` to Migration.DSL
Added docs for DSL's existence functions