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.

Use `boundp`  to check if `nrepl-send-sync-request` is exists?