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.
@stefanhoth  the reason is that if you just have it in the comment, people who are currently using this constant won't get notified by their IDE that it's deprecated. That means when it does get removed (which should be the intention when deprecated) eventually, then they'll have had no warning - might as well remove it straight off the bat now. (don't :tongue: )