Add all_addresses property

The all_addresses property should return all addresses currently
associated with the current account.
Calculate transaction fee

Instead of using the faucet.txfee value as an estimate of the total TX
fees, use it the set the per-kB fee amount. Then estimate the TX size,
as per the final formula here:

http://bitcoin.stackexchange.com/a/3011
Make get_available() method more conservative

Instead of using just the current pending payouts to calculate
transaction fees in get_available(), assume that we're in the middle of
adding a payout and calculate the tx fee by adjusting the tx output
count accordingly.