Fixed ac-cider-popup-doc and modified readme.

The function `nrepl-send-sync-request` is renamed into `nrepl-send-request-sync` in cider 0.7.0

https://github.com/clojure-emacs/cider/blob/v0.7.0/nrepl-client.el#L723-744

And maybe some one want to use `ac-cider-popup-doc` to popup doc instead of `cider-doc`, so i modified the readme.

@killme2008 So it appears that `nrepl-send-sync-request` is a latest version. If you update CIDER it should work for you. Can you please try?