make py lib a self-contained directory again

- move and merge _py/ bits back to py/
- fixes all around

--HG--
branch : trunk

testing/plugin/test_pytest_hooklog.py

@@ -0,0 +1,12 @@
+import py
+
+def test_functional(testdir):
+    testdir.makepyfile("""
+        def test_pass():
+            pass
+    """)
+    testdir.runpytest("--hooklog=hook.log")
+    s = testdir.tmpdir.join("hook.log").read()
+    assert s.find("pytest_sessionstart") != -1
+    assert s.find("ItemTestReport") != -1
+    assert s.find("sessionfinish") != -1