Split savepoints and transactions into separate interfaces
Object to manage nested transactions
Move isolation level to a separate interface

The most common implementation changes the isolation level for all
subsequent transactions. Unfortunately, the syntax for such behavior
wasn't standardized until SQL:1999 and most vendors ignore it.
PDO supports transactions
Require PHPUnit and DbUnit
Travis CI: Install DbUnit

This selects the latest stable version of DbUnit. PHPUnit is already
installed, though PEAR doesn't think so.
Merge branch 'feature/transactions' into develop

Closes GH-19