UPDATE transaction statements

Added UPDATE lightweight transactions according to issue #262 
Simply 
model.iff(col=value).update(col2=value2)
or
model.col2 = new_value
model.iff(col=value).save()

I believe I fixed the issues with my previous pull request #282 .  Sorry, if I'm a little slow on the uptake.  Still very new to the open source world.