Feature request: list both last payment and next payment time

[ironstru]

With the deterministic masternodes now we always have an block number estimate for the payouts.
It would be nice to have that information in the API too.

We could have:
MasternodeLastPaidBlock - block number when the last payment was received
MasternodeLastPaidTime - unix time when the last payment happened
MasternodeLastPaidAmount - the amount of the last payment
MasternodeNextPaymentBlock - block number when the next payment is due
MasternodeNextPaymentTime - an estimation when that block will happen based on previous block times