remove shutdown logic from PluginManager and add a add_cleanup() API

for the already existing cleanup logic of the config object.
This simplifies lifecycle management as we don't keep two
layers of shutdown functions and also simplifies the pluginmanager
interface.

also add some docstrings.

--HG--
branch : plugin_no_pytest

_pytest/assertion/__init__.py

@@ -70,12 +70,11 @@ def pytest_configure(config):
     config._assertstate = AssertionState(config, mode)
     config._assertstate.hook = hook
     config._assertstate.trace("configured with mode set to %r" % (mode,))
-
-
-def pytest_unconfigure(config):
-    hook = config._assertstate.hook
-    if hook is not None and hook in sys.meta_path:
-        sys.meta_path.remove(hook)
+    def undo():
+        hook = config._assertstate.hook
+        if hook is not None and hook in sys.meta_path:
+            sys.meta_path.remove(hook)
+    config.add_cleanup(undo)
 
 
 def pytest_collection(session):