Protocol time

`PaymentProtocol.createPaymentRequest` was using milliseconds for the `time` field of `PaymentDetails` rather than seconds as per [BIP-70](https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki).  On the other hand, [`PaymentSession.getDate()`](https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/com/google/bitcoin/protocols/payments/PaymentSession.java#L252) is correctly interpreting the value as seconds, resulting in significantly inaccurate creation times being reported for a `PaymentRequest`.