Re-enable recursive remote functions in a limited form. (#453)

* Re-enable recursive remote functions in a limited form.

* Fix linting.

test/recursion_test.py

@@ -0,0 +1,26 @@
+# This test is not inside of runtest.py because when a recursive remote
+# function is defined inside of another function, we currently can't handle
+# that.
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import ray
+
+ray.init()
+
+
+@ray.remote
+def factorial(n):
+  if n == 0:
+    return 1
+  return n * ray.get(factorial.remote(n - 1))
+
+
+assert ray.get(factorial.remote(0)) == 1
+assert ray.get(factorial.remote(1)) == 1
+assert ray.get(factorial.remote(2)) == 2
+assert ray.get(factorial.remote(3)) == 6
+assert ray.get(factorial.remote(4)) == 24
+assert ray.get(factorial.remote(5)) == 120