Integrate FindBugs into maven build

A recent bug in XTS recovery was found to be caused by the use of == rather than .equals() for String comparison. This type of bug can go unnoticed for a long period of time as == will usually return true for Strings with identical contents. However, it is not guaranteed.

We should integrate FindBugs (http://mojo.codehaus.org/findbugs-maven-plugin/) into our build for the Narayana project. FindBugs should run in the test-complie phase.