Ability to read and interpretate the last session's entries of a HellaNZB log file

We currently have only one case where it's necessary to look into a HellaNZB log file: In modes.py; to detect whether a running HellaNZB instance is using the correct configuration file.

The implementation is somewhat crappy to be honest. Since we now have the possibility to give HellaNZB log messages a meaning and to extract information, why not implement something based on this new infrastructure? Other problems and features might arise in the future that make it necessary to know the content of a HellaNZB log file. For the moment, it's certainly a nice-to-have functionality.