Tolerate failures trying to roll the journal file.

The motivation for this feature is to handle a scenario where a system might hit its file descriptor limit and be unable to roll the journal file. We would like to handle this scenario by continuing to write to the current journal file rather than failing immediately. 

The idea is to allow prevayler to keep operating until some file descriptors can be freed up and the journal file can eventually be rolled. The feature is disabled by default.

I have added easymock and powermock as dependencies to help me test this feature. If you don't think this feature could be helpful to others or don't want to maintain it, that's fine, we'll just maintain it ourselves on our own fork. Suggestions, corrections and criticism are welcome.