Polish changes following #3305

Also add support for field comments in PostgreSQL & Oracle ... only via alter_table() for now :/
diff --git a/system/database/drivers/oci8/oci8_forge.php b/system/database/drivers/oci8/oci8_forge.php
index 433692d..3576a9c 100644
--- a/system/database/drivers/oci8/oci8_forge.php
+++ b/system/database/drivers/oci8/oci8_forge.php
@@ -106,6 +106,14 @@
 			else
 			{
 				$field[$i]['_literal'] = "\n\t".$this->_process_column($field[$i]);
+
+				if ( ! empty($field[$i]['comment']))
+				{
+					$sqls[] = 'COMMENT ON COLUMN '
+						.$this->db->escape_identifiers($table).'.'.$this->db->escape_identifiers($field[$i]['name'])
+						.' IS '.$field[$i]['comment'];
+				}
+
 				if ($alter_type === 'MODIFY' && ! empty($field[$i]['new_name']))
 				{
 					$sqls[] = $sql.' RENAME COLUMN '.$this->db->escape_identifiers($field[$i]['name'])