Improve account get_balance and get_credit_debit
If we add the constraint on parent of account to be in the same company,
we could use MPTT directly in the SQL query (as in [1]).
It is because in this case, every children account will be in the same
currency. The constraint was not there to allow to construct some kind
of consolidation of multi-company account chart. But I think it is a
wrong design, this will be achieve with a new kind of report tree.
[1] http://bugs.launchpad.net/openobject-addons/+bug/887376