Restrict defined_names request

- Do not call it during auto-completion.  AC changes text property
  a lot and after-change-functions hook is fired.  As the server
  is busy replying completion requests, do not call defined_names.

- Request defined_names one at the time.  Same technique as
  get-in-function-call is used.
Document imenu support
Forgot to add autodoc for jedi:install-imenu