Add #getPrefix to InterledgerAddress.java

[sappenin]

It would be useful to have InterledgerAddress be able to provide the prefix for an address that is not itself already a prefix. Something like the following:

final InterledgerAddress getPrefix(){
  if (this.isLedgerPrefix()) {
    return this.getValue();
  } else {
    return InterledgerAddress.of(getValue().substring(0, this.getValue().lastIndexOf(".")));
  }
}