streamline pytester API majorly:

- integrate conftest into pytester plugin
- introduce runpytest() to either call runpytest_inline (default) or
  runpytest_subprocess (python -m pytest)
- move testdir.inline_runsource1 to pdb tests
- strike some unneccessary methods.
- a new section "writing plugins" and some better pytester docs

--HG--
branch : testrefactor

testing/test_doctest.py

@@ -1,5 +1,5 @@
 from _pytest.doctest import DoctestItem, DoctestModule, DoctestTextfile
-import py, pytest
+import py
 
 class TestDoctests: