Ported TransactionSigner.
Also had to change CustomTransactionSigner, DummySigSigner, LocalTransactionSigner as they were accessing val of TransactionSigner.ProposedTransaction.
Note that we need to call valMemberName() to access a val member whose name is valMemberName on a Scala class.
See Vals at https://twitter.github.io/scala_school/java.html
mvn test passed.