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`.

PS. I don't know why this pull request is showing 14 commits when there should be only one.  Obviously I'm not understanding git correctly.  If someone can tell me what I'm doing wrong I'll stop doing it.