- threadlocal engine wasn't properly closing the connection

- threadlocal engine wasn't properly closing the connection
upon close() - fixed that.
- Transaction object doesn't rollback or commit if it isn't
"active", allows more accurate nesting of begin/rollback/commit.
- Added basic support for mxODBC [ticket:1710].
- Python unicode objects as binds result in the Unicode type,
not string, thus eliminating a certain class of unicode errors
on drivers that don't support unicode binds.