#1 statementdifference defined incorrectly

[sherif]

statementdifference definitions in
http://www.adempiere.com/wiki/index.php/ManPageW_BankStatement
and http://www.adempiere.com/wiki/index.php/ManPageW_CashJournal
do not match the implementation. In the implementation, it is maintained as
Difference between statement/Cashbook beginning and ending balance (Sum of lines)
Also see https://sourceforge.net/forum/message.php?msg_id=6749295

I also think there is a need to maintain a column which does what the current
ManPage says, at least in CashJournal. Will open a separate tracker for that