[Emacs mode] Add completion by auto-complete.el.

According to comments at #113, I separated the code that depends on auto-complete.el.

If `tern-ac-on-dot` is non-nil (default), typing `.(dot)` invokes auto-complete with tern.
Calling the command `tern-ac-complete`, one can invoke auto-complete manually.
This program does not provide an ac-source for arbitrary timing yet.

### Installation

Add following lines below the tern setup code.

```lisp
(eval-after-load 'tern
   '(progn
      (require 'tern-auto-complete)
      (tern-ac-setup)))
```