Added #112 sign with cosignatories

Author: rg911

test/model/transaction/CosignatureTransaction.spec.ts

@@ -15,6 +15,7 @@
  */
 
 import {expect} from 'chai';
+import { VerifiableTransaction } from 'nem2-library';
 import {CreateTransactionFromDTO} from '../../../src/infrastructure/transaction/CreateTransactionFromDTO';
 import {Account} from '../../../src/model/account/Account';
 import { NetworkType } from '../../../src/model/blockchain/NetworkType';