Transaction management for Structure

[lsbardel]

A design choice needs to be made when pulling data from stdnet.orm.Structure. Currently there is no transaction. Data is pulled immediately.

This is in contrast with the sessions commit which can be performed using transactions

s = session.add(Set())
with session.begin():
    t.update((2,3,4,56))
    t.delete((89,78))

It is the quivalent of a stdnet.orm.Query.