Make transaction handling pluggable

Currently, the transaction management cannot be changed by the user on a general basis (i.e. it is possible to handle transactions totally outside Torque by passing a database connection into the methods, but there is no place to plug in an own transaction management into torque).This makes using external transaction management systems (e.g. Spring's @Transactional annotation) difficult.
To achieve pluggability, the Transaction class should pass all method calls to an implementation class which can be exchanged by an entry in the Torque runtime configuration.