cleanup py.test.* namespace, docstrings for improved pydoc and interactive usage.

use new apipkg __onfirstaccess__ feature to initialize the py.test namespace with the default plugins.  This, besides other good implications, means that you can now type:  pydoc py.test   or help(py.test)

--HG--
branch : trunk

testing/pytest/test_pluginmanager.py

@@ -223,6 +223,7 @@ class TestPytestPluginInteractions:
             import py
             def test_hello():
                 assert hello == "world" 
+                assert 'hello' in py.test.__all__
         """)
         result = testdir.runpytest(p) 
         result.stdout.fnmatch_lines([