Fix to driver to handle errors correctly

Viacheslav Tararin found an error where the driver was not correctly
handling multiple errors on the same connection. I have modified
QueryExecuter so that it now will simply accumulate errors on the
backend until the protocol has finished, and then throw the exception.

I have updated the jar on the download site as well.

I have also done a little more work on the logging mechanism. You can
now specify the logLevel with

org.postgresql.Driver.setLogLevel(DEBUG,INFO,WARN,ERROR,FATAL)

I'm loath to start sprinking debugging messages around, since it will
only slow the driver down. But the mechanism is there

Cheers,

Dave