add interface of pos-tip.el

I want to use pos-tip.el for popping up guide.
So, I've added the option to let user select the interface of popping up guide.
That is `guide-key/popup-function` variable, which is set `guide-key/popup-function` in default.
User can use pos-tip.el by the following config.

```lisp
(setq guide-key/popup-function 'guide-key/pos-tip-show)
```

![guide-key-demo1](https://f.cloud.github.com/assets/3935754/2242216/45fd4b62-9cfb-11e3-9c8d-99c0a3527e7d.png)

In addition, user will be able to use guide-key.el in mini-buffer by this feature.

![guide-key-demo2](https://f.cloud.github.com/assets/3935754/2242266/db417260-9cfc-11e3-9f46-d9832852fad1.png)

Best regards.