Log real-time index updates

Without logging the REPLACE queries on the real time indices, it was difficult to see if things were being updating as they were supposed to.

This PR adds a log call to `ThinkingSphinx::RealTime::Transcriber#copy`. 

It also extracts the logging logic into a common `ThinkingSphinx::Logger.log` method, which is then used by all objects that need to logging. I'm not entirely sure about this design; maybe a `ThinkingSphinx::Logging` module with a `log` method that gets included on classes that need logging is a better approach. Suggestions are welcomed! :smiley_cat: