Added 'winheight' saving/restoring.

Hi, Paradigm.

**Why?**

I use a lightweight version of the "Rolodex" concept: `set winheight=999`.

Because SkyBison assumes the default value for `winheight` and arbitrarily sets the height of the useful part of the plugin's window we end up in a weird situation:

![original behavior](https://f.cloud.github.com/assets/344335/1499904/f04ca0ac-4866-11e3-9918-f486985bf95e.png)

**How?**

I've added three lines to the existing logic that:

* save the value of `winheight`
* set it to `1` (the default value)
* restore it to its initial value upon quit

Result:

![modified behavior](https://f.cloud.github.com/assets/344335/1499913/23aba72c-4867-11e3-8011-a9ca794a6615.png)

I'm aware that those changes are very specific (to me) but they seem relatively harmless.