[zfcampus/zf-apigility#56] Do not return a value!

Returning a value can be used by the object triggering the event to stop
propagation; as such, any listener that is listening on wildcard events, or,
typically, more than one event, likely should not return a value.

In this particular case, the logging listener was returning a boolean true
value. This broke the zf-mvc-auth "authorization" event, as boolean return
values can be used to short-circuit (they indicate authorization status).

The code inside this method also indicated that the return value was a hack
until a ZF2 issue was resolved -- and it has been for some time now.
Merge branch 'hotfix/zf-apigility-56'

Close #157
Fixes zfcampus/zf-apigility#56
[#158] return a truthy value

- Per discussion with @Ocramius, a `null` value does not work currently
  until zendframework/zf2#6147 is in a released version. That patch
  fixes a `!$value` call to more correctly be a `false === $value` call;
  until then, a truthy, but not boolean true, return value will also
  still work.
Merge pull request #158 from weierophinney/hotfix/zf-apigility-56

#157 breaks compatibility with current Zend\Session