* Add tests for new subtree method

tests/test_subtree.py

@@ -0,0 +1,19 @@
+from __future__ import unicode_literals
+from spacy.en import English
+
+EN = English()
+
+
+def test_subtrees():
+    sent = EN('The four wheels on the bus turned quickly')
+    wheels = sent[2]
+    bus = sent[5]
+    assert len(list(wheels.lefts)) == 2
+    assert len(list(wheels.rights)) == 1
+    assert len(list(wheels.children)) == 3
+    assert len(list(bus.lefts)) == 1
+    assert len(list(bus.rights)) == 0
+    assert len(list(bus.children)) == 1
+
+    assert len(list(wheels.subtree)) == 6
+