#933 Add MySQL to SQLite converter

- Create common parent for converting from one datasource type to another
- Add MySQL to SQLite child
- Create tests

src/main/java/fr/xephi/authme/util/MigrationService.java

@@ -69,7 +69,7 @@ public final class MigrationService {
             try {
                 SQLite sqlite = new SQLite(settings);
                 ForceFlatToSqlite converter = new ForceFlatToSqlite(flatFile, sqlite);
-                converter.run();
+                converter.execute(null);
                 settings.setProperty(DatabaseSettings.BACKEND, DataSourceType.SQLITE);
                 settings.save();
                 return sqlite;