Make AttachmentsTable, AuthorsTable, and TagsTable classes instead of structs.

Frameworks/Database/AttachmentsTable.swift

@@ -10,7 +10,7 @@ import Foundation
 import RSDatabase
 import Data
 
-struct AttachmentsTable: DatabaseRelatedObjectsTable {
+final class AttachmentsTable: DatabaseRelatedObjectsTable {
 
 	let name: String
 	let databaseIDKey = DatabaseKey.attachmentID

Frameworks/Database/AuthorsTable.swift

@@ -17,7 +17,7 @@ import Data
 // CREATE TABLE if not EXISTS authorLookup (authorID TEXT NOT NULL, articleID TEXT NOT NULL, PRIMARY KEY(authorID, articleID));
 
 
-struct AuthorsTable: DatabaseRelatedObjectsTable {
+final class AuthorsTable: DatabaseRelatedObjectsTable {
 	
 	let name: String
 	let databaseIDKey = DatabaseKey.authorID

Frameworks/Database/TagsTable.swift

@@ -17,7 +17,7 @@ import Data
 // CREATE TABLE if not EXISTS tags(tagName TEXT NOT NULL, articleID TEXT NOT NULL, PRIMARY KEY(tagName, articleID));
 // CREATE INDEX if not EXISTS tags_tagName_index on tags (tagName COLLATE NOCASE);
 
-struct TagsTable: DatabaseRelatedObjectsTable {
+final class TagsTable: DatabaseRelatedObjectsTable {
 	
 	let name: String
 	let databaseIDKey = DatabaseKey.tagName