Log matching trigger/change filters

To aid in debugging configuration errors or perhaps bugs in the
filters themselves.  Will produce output like:

  2014-06-24 09:49:22,553 zuul.IndependentPipelineManager  DEBUG    Event <TriggerEvent comment-added org/project1 master 1,1 CRVW:2> for change <Change 0x7eff08059090 1,1> matched <EventFilter types: comment-added required_approvals: {'email': 'jenkins@example.com'}> in pipeline <IndependentPipelineManager trigger>

Change-Id: Iea5c14bf2532d854f6d3ad91de5e3e5537e8a431