Moving master to Pgobject

This moves Master to the PGObject framework.

For 1.5, the target is to rewrite the financial logic including payments and reconciliation.

I think 1.6 can shoot for getting rid of LedgerSMB::PGOld (replacement for LedgerSMB::PGObject).
Would be nice if there was a comment block enumerating what needs to be done to eliminate this file. Since it's a compatability wrapper, it probably has no long-term place in the code base.