Fix statement interruption

1. Fix syntax for interruptStatement method to refer to correct `conn` object.

1. Fix SQL statement for interruptStatement to use `statementID` variable (not SQL column).

1. Add unit test to verify statement interruption.

Coffeescript modifications only. Javascript not built.

*NOTE:* Unfortunately, the normal testing method using `SELECT SLEEP(1000)` does not work since `SLEEP` statements are not interruptible. See connection_test.coffee for more comments.
This test succeeds while using the SLEEP because you're not attempting to interrupt the statement, only interrupt the connection. The connection is dropped, but as far as I know, the command still runs.

So, it's important to note that interrupting the connection will not necessary interrupt a query. It