logger: added conditional logging via 'skip' function in options.

This came about because of this StackOverflow question: http://stackoverflow.com/questions/21030885.  The OP wanted to be able to log conditionally based on the response status code.  I chose to implement this by testing for the presense of a function called 'skip' in the options object.  If that function is there, it is invoked with the request and response: if its result is truthy, the request is not logged.  Simple and low-impact.