Improvement of memory management for #678

[victor-cr]

Avoiding recreating logger for the namespace which has been already created. Fix memory leak for a fixed number of categories. For dynamic categories method destroy is still required. #678

[coveralls]

Coverage increased (+0.4%) to 88.123% when pulling b0026ed on victor-cr:master into 5c7c61d on visionmedia:master.

[coveralls]

Coverage increased (+0.5%) to 88.168% when pulling 54d4acc on victor-cr:master into 5c7c61d on visionmedia:master.

[randycoulman]

Thanks for this PR! I was thinking of submitting a similar one myself, as we just discovered that this memory leak was hitting us hard.

I was thinking about using a Map instead of an object for the instances. Would that be a better choice?

Map seems like the more suitable data structure for this usage and would simplify the code in destroy and enable, but I'm not sure of the relative performance characteristics, or whether Map is available in every target environment.

[victor-cr]

I have no objection of using Map - the reason I was using object just to use as simple entity as it is possible. Also, destroy and enable are rarely used in comparison to logging.