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/confrest.py

@@ -1,5 +1,5 @@
 import py
-from py.__.rest.resthtml import convert_rest_html, strip_html_header 
+from _py.rest.resthtml import convert_rest_html, strip_html_header 
 
 html = py.xml.html