generalize skipping

- rename pytest_xfail to pytest_skip
- dynamic "skipif" and "xfail" decorators
- move most skipping code to the plugin

also coming with this commit:
- extend mark keyword to accept positional args + docs
- fix a few documentation related issues
- leave version as "trunk" for now

--HG--
branch : trunk

testing/pytest/conftest.py

@@ -1,3 +1,3 @@
 
-pytest_plugins = "pytest_xfail", "pytest_pytester", "pytest_tmpdir"
+pytest_plugins = "skipping", "pytester", "tmpdir"