Allow to use an alternate transaction manager class.

By using a 'txn_manager_class' argument in the Model constructor, it is
now possible to have Teng instantiate another class than the default
DBIx::TransactionManager.

This change is totally invisible for the end users and should not break
any existing code.
Merge pull request #126 from GeJ/alternate_txn_manager

Allow to use an alternate transaction manager class.