Update nginx config to reflect best practice

Should use `try_files` then fall back to a named location block. This is also what's recommended inside the docs already: http://gunicorn-docs.readthedocs.org/en/latest/deploy.html#nginx-configuration
Consistent usage of `@proxy_to_app` named location

This can be confusing since `location / {}` and `location @proxy_to_app {}` were both being used.
Merge pull request #763 from mattrobenolt/patch-2

Update nginx config to reflect best practice
Merge pull request #764 from mattrobenolt/patch-1

Consistent usage of `@proxy_to_app` named location
add on_exit hook
sync documentation

generally speakiing people shouldn't update the settings.rst but make
sure that settings.p is correctly filled.
save listener socket name so we can handle buffered keep-alive requests *after*
listener has been closed (i.e. stopped accepting during worker teardown)