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

This patch adds two commands: `tern-ac-complete` and `tern-ac-dot-complete` and an option variable: `tern-ac-on-dot`.

![screenshot](https://f.cloud.github.com/assets/158801/463378/9e74d5aa-b555-11e2-84b7-952bb129f8c8.png)

This modification includes an ad-hoc state value for the variable `tern-activity-since-command`, which is asynchronous flow controlling. I'm not sure that this is valid code.