Python 3 compatibility. (#121)

* Make common module Python 3 compatible.

* Make plasma module Python 3 compatible.

* Make photon module Python 3 compatible.

* Make numbuf module Python 3 compatible.

* Remaining changes for Python 3 compatibility.

* Test Python 3 in Travis.

* Fixes.

test/array_test.py

@@ -7,6 +7,10 @@ import ray
 import numpy as np
 import time
 from numpy.testing import assert_equal, assert_almost_equal
+import sys
+
+if sys.version_info >= (3, 0):
+  from importlib import reload
 
 import ray.array.remote as ra
 import ray.array.distributed as da