Add configurable Moving Average param to AuctionHouse.sol

[willyogo]

As ATX DAO, we want to be able to set a Moving Average Period in order to smooth out the volatility of auction durations. If the MA Period is set to 1, then only the last auction's sale price will be used to calculate the next auction's duration. if the MA Period is set to X, the salePrices from the last X auctions will be averaged to calculate the duration of the next auction. The greater the moving average length, the less volatility there will be in auction duration between auctions.

1. Owner of the AuctionHouse.sol contracts (ie. atx dao treasury in prod or deployer of contracts in staging) can set a Moving average period
2. Moving average period sets the number of auctions that will be averaged out.
3. MA period must be > or = 1
4. MA period is set to 2 after deployment