Ignore any unused entry that starts with '_'.

In addition to '_', also ignore any unused variables whose name start
with 'unused', as recommended by http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Lint#Lint

Add mman.pxd and wait.pxd

--HG--
extra : transplant_source : 7%EC%04%1F%89%F6%AA%8C%7Fl%B5D%DA%D5%CAF%B7%A70%0D

expand doc section on cython -a output

* explain new "+" signs in front of lines
* explain different shades of yellow
* relation to nogil blocks

Fix caching issues of cython magic.

Should fix https://github.com/ipython/ipython/issues/6507 when using cython magic. 
We will deprecate cython magic in IPython itself.