Really disable blammo if no configuration is specified

This Pull Request modifies Logger's behavior:
- it makes Logger.level() return Levels.OFF if no level is set. Previously, this returned undefined.
- it disables Logger.traceEntry() and Logger.traceExit() if Logger.level() < Levels.TRACE.

Together, these two commits make sure that no log will take place if blammo isn't configured, thus not impacting the application's performance.

Here is my specific use case : I have an application that uses SaXPath, which itself uses blammo. I don't want any blammo logging operation to occur, as it really slows down my application.