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 xyz

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.
while you're in this class, can you move the deprecated tag here so it's below the javadoc and above the method declaration please :dancer: