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

doc/test/funcargs.txt

@@ -276,6 +276,7 @@ methods in a convenient way.
 .. _`conftest plugin`: customize.html#conftestplugin
 
 .. _`funcarg factory`:
+.. _factory:
  
 funcarg factories: setting up test function arguments 
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