Add support for "currentSchema" connection property (in 9.3 branch).

Following the discussion in #156, here's a pull request targeted at the 9.3 branch.
I simply backported alexismeneses' code but I wasn't sure what to do in regard to JDBC 4 [g|s]etSchema() API (should it be supported or not?).
And also I didn't include the unit test for now, which depends on the previous question.