updates to tutorial, modified logging behavior

-updates to planout tutorial
changes to planout:
 -calls to log_exposure() always log data
 -added get_segment() method to make Namespace class more friendly, in
terms of what’s going on underneath the hood.
 -bug fix to enable disabling logging at experiment level
 -added log_exposure() and log_event() to namespaces
 -made it so that namespaces log to a file with the same name as the
experiment.