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.
:+1: