Mark Do()/With() transaction as managed.

Transaction created from Do() and With() are now considered "managed".
Managed transactions cannot be manually committed or rolled back since
the Do() and With() functions provide that functionally automatically.
Previously, a Tx could be manually committed and then any changes after
that would be lost.
Merge pull request #78 from benbjohnson/tx-managed

Mark Do()/With() transaction as managed.