Failing test for nestedtransactions.

A repro of https://github.com/ServiceStack/ServiceStack.OrmLite/issues/110
Fix for nested transaction handling:
 If the connection is an OrmLiteConnection, store the transaction in a new property when starting a new transaction. And use that property when creating dbCommands.
Merge pull request #129 from AkosLukacs/master

Multiple different transaction cause exception on SqlServer