Savepoints

[lsmith77]

initial version ..
a couple of questions:

• how to handle the savepointFormat config option?
• should the [create|release|rollback]Savepoint() methods check if they are supported?
• obviously since with savepoints we can actually do proper partial rollbacks, i am not setting the $this->_isRollbackOnly = true;
• don't really like the changes i had to do to the test suite

also of course there needs to be more tests, not only for the new savepoint setting, but also for [create|release|rollback]Savepoint()

[lsmith77]

main issue remaining is DB2 being stupid