Provide getter/setter for NoTils/simple.Log.SHOW_LOGS switch

This is mainly to get a way to avoid the findbugs warning:

    Write to static field com.novoda.notils.logger.simple.Log.SHOW_LOGS from instance method

Eventually this public static field should be demoted to private (breaking all projects in the process). Adding a deprecated flag for now to push people to migrate to the new accessor methods.
Found a useful check for this: ![IntelliJ Setting](https://www.dropbox.com/s/sjjyw2szbxxlw9f/shot_140724_135704.png?dl=1)