Incorporated lptBuyOrder management

[chrishobcroft]

Some comments:

• I renamed claimCollateralAndPayment to claimSellOrderCollateralAndPayment for the LptSellOrder management side of things, because it needed to be differentiated from the equivalent function on the LptBuyOrder management side, which I called claimBuyOrderPaymentAndCollateral. I considered reclaimSellOrderCollateralAndClaimPayment and reclaimBuyOrderPaymentAndClaimCollateral but found them too long-winded.

• I did not add tests, but if you want to write them (which I hope you do), then I would be very grateful if you would submit them as a pull request for me to review.

• I'm sure I'm missing a require somewhere, or I got something mixed up.