[IMP] account: support for partial reconciliation in aged partner balance

When computing the aged partner balance, the partial reconciliation was not
handled correctly. The reconciled amount should be removed from the original
remaining amount instead of displaying two entries in the journal.
eg: if invocie of 1000 in period 1 and payment of 300 in period 2, should only
display +700 in period 1 instead of two entries
Merge pull request #124 from odoo-dev/7.0-aged-partner-partial-mat

[IMP] account: support for partial reconciliation in aged partner balance
    
When computing the aged partner balance, the partial reconciliation was not handled correctly. The reconciled amount should be removed from the original remaining amount instead of displaying two entries in the journal.
eg: if invocie of 1000 in period 1 and payment of 300 in period 2, should only display +700 in period 1 instead of two entries