For exact swap there is always dust amount

[top1st]

@daoio @bghughes Can you please review this issue?

The fee logic makes always dust and it will keep make external gas out also users will not happy with not exact matching amounts.

Current fee logic made fees all from taker and make dust amount for exact matching request.

bytes32 makeId = market.make(token1, token2, 1 ether, 1 ether);
market.take(makeId, 1 ether);

Offer ID: 1
pay_amt 1000000000000000000
buy_amt 1000000000000000000
Will Spend 1000000000000000000
quantity 999900000000000000
buy amt 1000000000000000000
pay amt 1000000000000000000
Internal Spend 999900000000000000
Maker Token 1 0
Maker Token 2 999900000000000000
Taker Token 1 999900000000000000
Taker Token 2 10000000000
Offer ID: 1
pay_amt 100000000000000
buy_amt 100000000000000

Here is some logs I made and always dust as fee percent.

I think need to upgrade the RubiconMarket contract subtract the quantity and spend amount as user take, buy function amount then make zero the left amount. after that need to remove the offer from the map.