Balance column and rescan revamp

A new Balance column is added to the transaction view. The
Balance column makes the transaction view look more like
a bank statement, showing the effect on your total balance
each transaction makes.

In order to get the Balance column to calculate the correct
balance, I needed to beef up the "-rescan" command line
option. There seem to be allot of bogus transactions in the
wallet that don't get removed by the old rescan. The new
rescan purges all transactions from the wallet. The new
rescan also runs as a thread which uses the QT progress bar.
New progress messages go to the debug log for the benefit
of a non-QT yacoind build.

So, if your transaction view balance does not match your
overall balance, then use the new and improved "-rescan" to
fix it.