add "reset" method

[gkorland]

"reset" method that can be overridden is a very important addition that will let a "resting" a resource before putting it back to the pool. The resource shouldn't be available until it was "rested" and should be released on err.

var poolModule = require('generic-pool');
var pool = poolModule.Pool({

    reset: function(resource, done) { 
          resource.clean(function(err){}{ 
             done(err);
          });
    }

}

[fbtariq]

Any update on this? Need to run some code after a resource is released to basically "clean" it before it is used again.