Introduced findbugs
fixed a potential NPE found by findbugs
Improved the filter to report serialization related issues

Couldn't figure out how to add specific checker to the default setting, so I'm activating a whole bunch by setting threshold=low, then excluding most of them except the serialization problem
Excluding warnings

It'd be good to fix the "collection of URL" problem but that's for
another day.
These classes did not have serialVersionUID when they should have.

Assigning the current values as the explicit value going forward, to maintain backward compatibility.