[engine] Add initial version of event loop engine

Re-add the engine that is responsible for tying together input, output,
etc.

see #23

tests/test_engine.py

@@ -0,0 +1,13 @@
+# pylint: disable=C0103,C0111
+import unittest
+
+from bumblebee.engine import Engine
+
+class TestEngine(unittest.TestCase):
+    def setUp(self):
+        self.engine = Engine(None)
+
+    def test_stop(self):
+        self.assertTrue(self.engine.running())
+        self.engine.stop()
+        self.assertFalse(self.engine.running())