Findbugs fixes and improvements

A series to:
- improve the ant build
- align findbugs to the bndtools setup
- upgrade findbugs
- fix a few findbugs warnings


Note:

Complation with ant fails on JDK 1.7 (at least for me) because of the osgi.cmpn jar, see
https://www.osgi.org/bugzilla/show_bug.cgi?id=153.

I compile the project using bndtools in Eclipse, which does work (surprisingly),
and then run findbugs etc.