[rllib] Add n-step Q learning for DQN (#1439)

* n-step

* add sample adjustm

* Oops

* fix nstep

* metric adjustment

* Sat Jan 20 23:30:34 PST 2018

* Sun Jan 21 16:40:46 PST 2018

* Mon Jan 22 22:24:57 PST 2018

python/ray/tune/trial.py

@@ -42,6 +42,9 @@ class Resources(
         return super(Resources, cls).__new__(
             cls, cpu, gpu, driver_cpu_limit, driver_gpu_limit)
 
+    def summary_string(self):
+        return "{} CPUs, {} GPUs".format(self.cpu, self.gpu)
+
 
 class Trial(object):
     """A trial object holds the state for one model training run.