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/plugin/test_pytest_mark.py

@@ -1,10 +1,10 @@
 import py
-from py.plugin.pytest_mark import Mark
+from py.plugin.pytest_mark import MarkGenerator as Mark
 
 class TestMark:
     def test_pytest_mark_notcallable(self):
         mark = Mark()
-        py.test.raises(TypeError, "mark()")
+        py.test.raises((AttributeError, TypeError), "mark()")
 
     def test_pytest_mark_bare(self):
         mark = Mark()