add 'zblog' mapper + tables + some object saves/gets to unit tests

[Mike Bayer]

the zblog demo tests out some patterns that we otherwise dont have represented in the unit tests, namely an elaborate map against a select statement that includes eager relations and backrefs. the whole suite should be made a module in tests with a bunch of data manipulations to make sure it continues working.

[Mike Bayer]

we've got a zblog unit test package in test/ which is a little minimal...but at this point SA has plenty of tests for the capabilities present in the zblog tests as well.